Custom form tag does not appear as available mail-tag

Hello,
We have created a custom form tag using the documentation at “Adding A Custom Form-Tag”. The tag works, in that the code gets rendered as a form field, and the form field gets incorporated into the mail template using the expected syntax.

However, the custom tag is not listed among the buttons in the form builder (the row of buttons that reads “text”, “email”, etc) – nor does the custom tag appear on the mail tab as a field that can be used. Form builders simply must know that the custom field exists.

I don’t see in the documentation any mention of a step to take to have the custom field listed in these locations. Is this documentation available anywhere? I’m trying to reverse engineer how other add-on plugins achieve this (most notably CF7 Conditional Fields), but was hoping this instruction existed already and I just missed it.

Thanks!