I want to add and agreement field to be mandatory whern the staff select one value from other LOV an

1- I had created one SIT with two fields

2- one Field is LOV (Transaction Type with three option)  and the Other Filed is Agreement (text box)

3- I had linked the SIT ot the self-service

4- I need one the staff select the first option to show him the second filed as mandatoty field and when the user select any of other option to hide the second field.

Please advise how I can implement this?

Thank you

Ammar Zeidan