Hi @alfie thomas ,
The Affected User isn't an enumeration in SCSM. Test The Affected User, same like the Assigned To User, is a relationship between Incident class (Work Item Classe) and the User class in SCSM.
Maybe this is helpful (line 22 contains the relationship class Work Item to Affected User):
https://github.com/abaumgarten42/SCSM_Useful_PSscripts/blob/main/Get-IncidentDetails.ps1
And in this script you can see how to create a relationship in SCSM (instead of Primary User relationship you have to use the Affected User relationship)
https://github.com/abaumgarten42/SCSM_Useful_PSscripts/blob/main/Set-PrimaryAndAssignedUser.ps1
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(If the reply was helpful please don't forget to upvote and/or accept as answer, thank you)
Regards
Andreas Baumgarten