Select gear wheel > Settings
Press "Client Defaults"
Activate this feature "Use tax agent as the contact on all forms" if you want this to be applied for all the clients.
Note: This feature will apply only for newly created clients (feature released date: 24 Aug 2021)
This is how it will appear in client's tax form -
For the client that you do not want to include, just navigate to the client "Settings > Contact Details" and manually untick "Use tax agent as the contact on all forms"
Below example will show that Doe Marie's "Use tax agent as the contact on all tax form" is currently activated (after I have ticked it through "Client Default" settings), I will untick this if I do not want her to use the said feature.