Sorry, we don't have conditionally required fields.
A workaround would be to use question skip logic. Create 2 identical fields - one required and one not required. If they select yes show the required one, otherwise show non required one.
Sorry I should have provided a link to info on how to do this.
This section of the help documentation does a good job of explaining how to setup skip logic for a field:
Note: skip logic is a paid only feature.