https://help.gosimpletax.com/hc/en-gb/articles/360000571120-Submitting-Tax-Returns-as-an-Agent
Click on SETTINGS
Indicate that your filing Tax Returns on behalf your clients
Enter your Practice Name, Agent ID, HMRC User name and SAVE
Click on EDIT to go back to the main screen
Click on VALIDATE MY TAX RETURN
Click YES to proceed
Enter your HMRC Password
Click on FINAL STEP: CLIENT APPROVAL