Required after select yes

good day
I would like to add a required field after select yes.
p.a.: would you have a discount Yes or No
If no, nothing is required and the visitor can send the form.
If yes, a uploadfield is after that required.

My question: Is this possible and if so, how can I config this?
Regards, from the Netherlands
Raymond

https://wordpress.org/plugins/si-contact-form/