I sometimes need Turkish characters, and I've found the html version of them that work in HTML5 and character set utf-8 are the following:
Ç - ç (Ç - ç )
Ğ - ğ (yumusak g, upper and lower case)
I - ı (undotted I, upper and lower case)
İ - i (dotted i, upper and lower case)
Ş - ş (S and s with a tail)
Ö, ö, Ü, ü work ok as they are.
Mind you, I have never tried using these in the Form builder, You'll have to check it. I'd think they will work in the text that is 'hard coded' in the form messages, but possibly not in any input submitted by end users.