How to Fill a Form Field With Drop-Down Menu

I would like to fill a web form with values (the html code is the one below) but I don't know how to select an item in two fields that have values within a drop-down menu. The two fields that interest me are "Categories" and "Language". Can anyone suggest me how to do it? Thanks!

<form id='video_form' method='post' action='/admin/addvideo'
 class='needs-validation' ><input id='save_request'
name='save_request'  type='hidden' value='1'> <div
class='container'><div class='row
justify-content-start'><div class='col-12
text-center'><h2>Add New Video </h2></div></div><div
class='row justify-content-start'><div class='col-12
text-center'>&nbsp;</div></div><div class='row
justify-content-start'><div class='col-3
text-right'>Title:</div><div class='col-9 text-left'><input
id='Title' name='Title'  type='text' class='form-control
form-control-lg' placeholder='Title' value='' required >
<div class='invalid-tooltip'></div></div></div><div
class='row justify-content-start'><div class='col-12
text-center'>&nbsp;</div></div><div class='row
justify-content-start'><div class='col-3
text-right'>Model:</div><div class='col-9 text-left'><input
id='Model' name='Model'  type='text' class='form-control
form-control-lg' placeholder='Model' value='' > <div
class='invalid-tooltip'></div></div></div><div class='row
justify-content-start'><div class='col-3
text-right'>Tags:</div><div class='col-9 text-left'><input
id='Tags' name='Tags'  type='text' class='form-control
form-control-lg' placeholder='Tags' value='' > <div
class='invalid-tooltip'></div></div></div><div class='row
justify-content-start'><div class='col-3
text-right'>Categories:</div><div class='col-9 text-left'>
<select class='form-control form-control-lg selectpicker'
multiple   name='Categories[]' id='Categories'> <option
value='1896'  >Anniversary &#38; Celebrations</option> 
<option value='1876'  >Brand</option>  <option value='1877' 
>Clean Energy Leader</option>  <option value='1893' 
>Documentary</option>  <option value='1882'  >Events and
Shows</option>  <option value='1879'  >Historical</option> 
<option value='1890'  >Intro</option>  <option value='1899' 
>Performance test</option>  <option value='1874' 
>Plants</option>  <option value='1880'  >PLM</option> 
<option value='1883'  >Product Intro</option>  <option
value='1881'  >Product Launch</option>  <option value='1898'
 >Product overview</option>  <option value='1884' 
>Services</option>  <option value='1894'  >3D
Animation</option>  <option value='1873'  >Technical
Animation</option>  <option value='1878' 
>Testimonials</option>  <option value='1895'  >Training
Material</option>  <option value='1892'  >Walkaround with
voice over</option>  <option value='1900'  >Walk-In</option>
 <option value='1897'  >Web Series</option>
</select></div></div><div class='row
justify-content-start'><div class='col-12
text-center'>&nbsp;</div></div><div class='row
justify-content-start'><div class='col-3
text-right'>Language:</div><div class='col-9 text-left'>
<select class='form-control'   name='Language'
id='Language'> <option value='INT'  >International (no
language)</option>  <option value='INB'  >English</option> 
<option value='IOO'  >Italian</option>  <option value='DOO' 
>German</option>  <option value='FOO'  >French</option> 
<option value='BFO'  >French for Belgium</option>  <option
value='EOO'  >Spanish</option>  <option value='POO' 
>Portuguese</option>  <option value='NLO'  >Dutch</option> 
<option value='DKO'  >Danish</option>  <option value='SOO' 
>Swedish</option>  <option value='NOO'  >Norwegian</option> 
<option value='POL'  >Polish</option>  <option value='UKR' 
>Ukrainian</option>  <option value='HUN' 
>Hungarian</option>  <option value='CHZ'  >Czech</option> 
<option value='GRK'  >Greek</option>  <option value='ROM' 
>Romanian</option>  <option value='BRL'  >Portuguese for
Brazil</option>  <option value='LAT'  >Spanish for
LATAM</option>  <option value='MEX'  >Spanish for
Mexico</option>  <option value='CHI'  >Chinese</option> 
<option value='INC'  >International Chinese</option> 
<option value='JPN'  >Japanese</option>  <option value='KOR'
 >Korean</option>  <option value='CSI'  >Russian</option> 
<option value='HIN'  >Hindi</option>  <option value='MMR' 
>Burmese</option>  <option value='TUR'  >Turkish</option>
</select></div></div><div class='row
justify-content-start'><div class='col-3
text-right'>Publication Year:</div><div class='col-9
text-left'><input id='PublicationYear'
name='PublicationYear'  type='text' class='form-control '
placeholder='Publication Year' value='' > <div
class='invalid-tooltip'></div></div></div><div class='row
justify-content-start'><div class='col-3 text-right'>Video
Length:</div><div class='col-9 text-left'><input
id='VideoLength' name='VideoLength'  type='text'
class='form-control ' placeholder='Video Length' value='' >
<div class='invalid-tooltip'></div></div></div><div
class='row justify-content-start'><div class='col-3
text-right'>Music Details:</div><div class='col-9
text-left'><input id='MusicDetails' name='MusicDetails' 
type='text' class='form-control ' placeholder='Music
Details' value='' > <div
class='invalid-tooltip'></div></div></div><div class='row
justify-content-start'><div class='col-12
text-center'>&nbsp;</div></div><div class='row
justify-content-start'><div class='col-3
text-right'>Location:</div><div class='col-9
text-left'><input id='Location' name='Location'  type='text'
class='form-control ' placeholder='Location' value='' > <div
class='invalid-tooltip'></div></div></div><div class='row
justify-content-start'><div class='col-3
text-right'>Director:</div><div class='col-9
text-left'><input id='Director' name='Director'  type='text'
class='form-control ' placeholder='Director' value='' > <div
class='invalid-tooltip'></div></div></div><div class='row
justify-content-start'><div class='col-3
text-right'>Production:</div><div class='col-9
text-left'><input id='Production' name='Production' 
type='text' class='form-control ' placeholder='Production'
value='' > <div
class='invalid-tooltip'></div></div></div><div class='row
justify-content-start'><div class='col-3 text-right'>Web
format:</div><div class='col-9 text-left'> <select
class='form-control'   name='WebFilename' id='WebFilename'>