You'll need a mutliple user plan to submit as an agent, this means a minimum of two paid users on your account.

Click on 'ADD / SWITCH' in the top right and switch to the 'MASTER USER'.

Once on the 'Edit profile' tab, select the option labelled 'I am an accountant / tax agent and wish to file tax returnson behalf of my clients', fill in the boxes that are then revealed and click SAVE 

Click on EDIT in the top right to go back to the main screen 

 
Following this, to proceed to submission, carry out the following:

Click on VALIDATE MY TAX RETURN 

Click YES to proceed 

Enter your HMRC Password 

Click on FINAL STEP: CLIENT APPROVAL