FileMaker build serial code from other 2 fields strings

I need to create one field in a table that is composed from the strings from other 2 fields Table1 - Field1 (text) - Field2 (text) - Field3 (text) <-- This should be the the text that is in Field1 together with the text that is in Field2 with an underscore in between them, such that: Field1 Field2 Field3 Steve Smith Steve_Smith Carla Bean Carla_Bean Field 3 should be updated when Field1 or Field2 are modified.