XF 2.2 Custom Thread Feld combine dropdown selection with text field

Mr Lucky

Well-known member
I want users to make a selection either from preset options for choice files OR a text box as a kind of "Other, please specify" field.

I could do this with two fields (one dropdown and one text), but only if neither were required because I want to require the user to either choose drop down or write what they want - but must be one or the other.

Does anyone know how I could achieve this?