Execute JavaScript in Safari - Web Form

Not having used the Keyboard Maestro ‘Execute JavaScript in Safari’ before, I find myself confused on how to extract the JavaScript portion of web page in order to use this option.

As an example, the following is the URL form I want to use Keyboard Maestro to populate and get the results back.
An example address to populate would be:
Address: 420 SE 23rd Ave
City: Minneapolis
Zip: 55455

I want to fill out the Address, City and zip, leaving everything else blank and hit the submit button. If I try and program Keyboard Maestro fill in fields with the ‘Select Safari Field’, it says there are none found, thus my thought I have to use the JavaScript.

Any suggestions?