Make "Other" option text field in Radio button required

i want to force the user to add text i the "Other" field if he chosen the "Other"

how can i do that ?

because sometimes he press other with adding text...

Also can i add other to the "Line Radio Buttons" from widgets? 

Unfortunately it is not possible to force the user to add text in the "Other" field if he chosen the "Other".

Also it is not possible to add Other options inside of Line Radio Buttons widget.

A possible workaround would be to add new radio button option "other" and to add new text field that would be set as required. Then you need to set conditional logic that will show/hide text field if "other" radio button is selected.

Here is a demo form I made http://form.jotformpro.com/form/51222653721952? 

More about conditional logic you can read in this articles:

http://www.jotform.com/help/196-How-to-Hide-or-Show-Multiple-Fields-at-Once-Based-On-a-Single-Conditional-Entry 

http://www.jotform.com/help/57-Smart-Forms-Using-Conditional-Logic

Hope this will help. Let us know if you need further assistance.