API Send On Behalf of Another User Error

I’m using the Developer Account to test out an integration using the Adobe Acrobat Sign API. I have been able to successfully send agreements via the API.

I’m now testing out the use of the:

x-on-behalf-of-user

header to send an agreement on behalf of another user. I have added a 2nd user to my Developer Account and have logged in as them and shared access to the admin user of the Developer Account. I followed the steps under the Basic Sharing option here:

https://helpx.adobe.com/sign/web/users/sharing-user-content/overview.html

When I attempt to upload the Transient document using the x-on-behalf-of-user header I’m 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 found another article that says that “For Send on behalf function, enable Advanced Account Sharing for the account so that users can give sending permissions to other users while sharing their account” when using the REST API. I’m not seeing that option in my Developer Account - should I have access to Advanced Account Sharing in the Developer Account or does this need to be enabled?

Thanks