You will need to obtain a NNID (NetNumber ID) from NetNumber directly.
To add a new NNID under Sinch, the NNID needs to be provisioned with the carriers. An LOA and form must be submitted to T-Mobile to request provisioning. The cost is $2000 USD. Reach out to your Sinch Account Manager, or contact Sinch if you have any questions.
Visit the Sinch 10DLC Product Page to learn more or if you need more help with 10DLC or have any questions, contact Sinch, who will be happy help!