How to auto populate time card field based on selection from another field?

Summary:

I have 2 fields on the time card A and B, I want to auto populate a selection in B based on A's selection.

Content (please ensure you mask any confidential information):

Field A is based on a time and labor value set, and field B is based on GL Organization value set. The first 5 characters of any selection for A is 5 digits that matches with a corresponding value in B. I want to auto-populate B with the first 5 characters of any selection in A. How can we do this?

Version (include the version you are using, if applicable):

Fusion 24B (11.13.24.04.0)