How to add client authorisations to your Agent Services Account
Introduction
Why You Need an Agent Services Account
An Agent Services Account is required to connect your software to HMRC, sign up clients, and manage their records. Client authorisation must be held within this account. This is separate from your Online Services for Agents account, however authorisations can be added from one to the other.
Steps to Add Authorisations
- Sign in to your Agent Services Account using the Government Gateway ID created when the account was set up.

- On the welcome page, select "Add existing Self Assessment authorisations to this account."

- You will see a list of client authorisations already added, including the Agent Code used and the date added.

- To add further client authorisations, select "Add Agent Code."

- Enter the Agent Code for the client authorisations you wish to add, then select "Continue."

- Sign in using the Government Gateway ID associated with the Online Services for Agents account for that Agent Code.

Confirmation
Once complete, you will receive confirmation that all client authorisations under that agent code have been successfully added. You can then proceed to sign up your clients for Making Tax Digital for Income Tax.

For more info please refer the below
https://www.youtube.com/watch?v=a6Ewg6NR0hU&t=16s