Hello there!
I would like to create a macro for sending emails with selected signature, i have assigned the signature for email account but can't choose it in KM.
Tell me please what i am doing wrong?
Oh, just noticed that it’s not supported in macOS Sierra 
I had the same issue.
Two ways to do it. (that I know of)
1 - while in the mail app, tab to the signature pop-up, enter then arrow down to the signature then enter to exit. (use KM to tab, enter, arrow, enter)
2 - in the system prefs/keyboard/shortcuts/app shortcuts you can add the mail app, Enter the name of your signature there and assign a keystroke to it. Then when you are in the mail app composing a mail, have KM ‘type’ the keystroke assigned to the signature and it will become the active signature.
cheers
Thanks
The easiest way is to add a signature into mail text from KM.