Hello there,
Thanks for reaching out! There is a button action for "Join Signal Group" if you were wanting to direct visitors to a group that you have in Signal. The "Join Signal Group" is in the Click to Chat section of the button action menu.

If you were trying to make a Signal button for a call to your number or user name, we do not have a dedicated Signal button action that is available currently (other than join signal group), though you can likely just use a Website URL button action and paste in a Signal link for the URL. I believe the current way to make Signal links would be like this for a phone number:
https://signal.me/#p/+12345678901 (put in your phone number instead of 12345....)
Or like this for a username based link:
https://signal.me/#eu/<characters> (put your username instead of "characters")
This way you can make a button that will link to your Signal when clicked. Hope that helps!