Contingent worker name to end with a string

Summary:

Hello all,

we require an autocomplete rule, preferably a field validation, for our HR-Specialists.

When adding a contingent worker, their last name must end with -External  

I already tried building a rule (Business Object: Person name, Field: Last Name), but something is missing.

Help is appreciated, thanks!

Code Snippet:

Company and Use Case Number:

Dürr IT Service GmbH

#6