How do I split 1 tag field into 2?

My title field contains the song title and artist seperated by an '_' and my artist field is empty. How can I get the artist text from the title field into the artist field? I tried split field by separator but that just removed the _ and artist.

To split the title into two tags you can use just this action

Guess values action.
Source format: %title%
Guessing pattern: %title%_%artist%

Thanks Mike