How to add a field which can list requester ?

How to add a field which can list requester ?

Hello,
I need to add a field where i can select any requester.
The purpose of this is : if a team lead call for let's say a service call. the requester would be the team lead but the "impacted user" is someone else. i though i could use the E-mail Id(s) To Notify field, but it does have a usage when the manager of the team lead need to be inform also.
Any idea anyone ?

Regards,
Martin

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