Need Assistance with Developer Account Advanced Sharing Permissions

Is there anyone from Adobe Acrobat Sign tech support monitoring these community forums? I would like to test out the ‘Send On Behalf of Another User’ feature using my Developer Account as part of a prototype integration I’m working on for a customer. It’s not clear if this feature is enabled for Developer Accounts as I keep getting this error:

{"code":"INVALID_ON_BEHALF_OF_USER_HEADER","message":"Value provided in x-on-behalf-Of-user header is in invalid format"}

I can’t seem to contact anyone at Adobe any other way except via this community forum but it looks like no-one is actually monitoring? Is there another way I can get an answer to my question?

Thanks