You have multiple options.
1. Access the Audit Values with any Action Button
dbFront will save field changes including Audit Value fields to the database before any action button is called. This means that you can use any appropriate action button type and you will also have access to the appropriate audit values.
The record save and the action button will run within the same transaction so everything is rolled back if there is an error.
2. Pass the %username% directly into the procedure
dbFront will allow you to pass the %username% value directly to the procedure as one of the parameters.