How give the admin system user in partner Business Manager permission for client's business?

I am following this guide Business On Behalf Of and in the step 2 Im getting this 400 error with the text Your access token doesn't have permission to access this client business. The access token is from the admin system user. How am I supposed to give permissions to the admin system user? which it was created in the partner account of which I am the administrator. I have not found documentation of how to do it. Also the client business belongs to a test user