You can add your Agent credentials to your Partner account. You can then enable a feature on each client account that will allow you to submit CIS returns and automatically verify subcontractors using these credentials.
Login to your Partner account and navigate to Settings > Organisation > Details & Tax.
Click on the Tax tab and fill in the HMRC sender ID and password (your agent services account credentials).
Next, login to a client account that you would like to either submit CIS returns or verify subcontractors.
Head to the Settings > Configure System > Toggle features menu and click on the Accounting tab and enable the Set accountant as Agent option.
You will now be able to automatically verify subcontractors on the Tax > CIS > Verification menu. Your agent credentials will also be used when submitting a CIS return.