There is an error in specifically the C# version of this activity, where the RelativeEndpoint property has a validation check that must be passed to remove all errors. A hardcoded value will pass without issues, but if you pass any variable in the string it suddenly is not longer valid.
The exact same input can be fed into a VB version of the activity, and will be valid there as expected.
This has been an issue since at least 2021: Orchestrator HTTP Request - How to use a variable in the RelativeEndpoint
I do not currently see a way to supply any variable into this request for C# until this issue is resolved.
error text: Value for the required activity argument RelativeEndpoint was not supplied in activity designer nor in project settings.
Edit: This was resolved somewhere after 2021, the current 2022.12 version does not have this error