Predefined Value problem

Hello,
I need to set the default value of a field to match the value of another field, but I’m unable to get it to work. It only works if I enter a specific number.

When I set the default value to the name of another field, it takes the numeric part of the field name as the value instead of the actual field value (e.g., fieldname38 becomes 38, but I need the value of fieldname38).

https://ibb.co/2SdQ2j0

I hope this is clear. How could I solve this?
Thank you.