Text entry – no entry trigger

Afternoon all

I have 4 text entry fields on a slide and a submit button. I want to have a layer stating you need to 'select an item' if the learner selects the 'submit' button if the learner hasn't entered any text into a text entry field. This would automatically happen if this was a free form question, however then are additional interactives that need to occur after the text entry input.

I had hoped to have the 'submit' button disabled and become active when the text input fields had text in them but I can't get this to work.

I hope this makes sense.

I look forward to your help.

Jen