@CraigGillgrass do you have any insight regarding this question? I'm having the same issue. It seems as though there is a bug - NIM102295 that might be contributing to the issue. It seems that if your editor tracking fields are already present in the feature layer, you can't enable the "Keep track of created and updated features" option, even though the fields are there to support it. Without this option checked, it seems you cannot add a feature level webhook to the assignment layer. You would think that if Workforce adds these "track edit" fields when the feature layer is created (when a new workforce project is started), the solution would be for workforce to ensure that the Change Tracking option is activated on that layer.
This is frustrating to me because I have another workforce project that I found a work-around for this last year and set up the feature level webhook, but I can't for the life of me remember what I did to get this enabled. I'll keep looking though. I can't believe the OP didn't get any response...