RTF for a text box or text area in form

With my Mac system I have a very interesting feature:

I can drag URLs from safari into text boxes or text file documents. Ok! I know this is not sooo new.... but, what is quite interesting is that if I have a textedit window in normal text and another one in RTF text,

when url (favicon) dragged to Normal Text editor I get the URL

and

when url (favicon) dragged to RTF text I get the page title, not the URL.

This is a great feature to add external links to products very fast.

The question is: How can I make my text box or text area custom field to be RTF type?

I found that in other pages this feature exists so it has to be a standard HTML feature but can't find the setting when creating new custom fields.