Setting field value based on another field?

Hello there,

We recently moved to YouTrack from OnTime and love it. One thing I did to help migration is create a custom migration program to move data over. We have a field, called OnTimeRelease that is a custom string field. What I want to do now, is for each issue migrated (I can filter it down to those), set the Fix Versions field value to be whatever is in the OnTimeRelease field. Ideally with some sort of verification but if not I can verify myself that the Fix Versions bundle has all values that the OnTimeRelease field has.

Any help doing this? It is a one time task FYI if that helps