Hi D T
Above email address might not have included you in allowed sender list (probably personal email address)
You can submit the details on the form itself. https://aka.ms/visionaipublicpreview and wait for feedback from concerned team.
Attached documentation on getting authorization to azure resources
Reference on using Azure Resources using Postman
Alternative approach is to use Python or any other SDK. You can also test in Cloud shell using Curl commands.
Hope it helps.
Thank you