How do I make a textfield required based on a particular radio button selection?

Hi,

I was wondering if I could get some help with making a textfield required based on a particular radio button selection.

I have a form that I'm creating and have several questions with radio buttons. For some of my questions there are yes/no radio buttons and if the user selects "yes", I ask "if yes, what?" with a textfield for their explanation beside it.

How do I make the below textfield required, only if, the user selects the "yes" radio button?

Thank any of you in advance for any help!

Name of radio button "Yes"= coy

Name of radio button group = Criminal_Offense

Name of textfield for "If yes, what? = whatinput