How to edit the source of additional fields in joomla dj-classifieds?

Question

Joomla

Greetings. There is an ad site on joomla with dj-classifieds. By default, you can add the ability to add a WhatsApp number (not a very nice-looking thing, by the way). I wanted to add the ability to also specify a link to instagram, for this I added a custom field in the dj-classifieds admin panel, but on the site it looks extremely unusable (instagram.com is displayed with the ability to change). Here, as I understand it, you need to edit the php of this field, but I can’t figure out where to find it. Even the search does not find "WhatsApp", nor "wa.me", nor "instagram" (a search based on the contents of the files did not return any results). Where, in what directories and files are these fields stored so that you can insert a non-editable link (hidden), as is it done with WhatsApp?