You need to navigate to Accounts Production >> Choose the client >> Settings >> Report Settings >> Company Info >> Edit, update the Trading status as Dormant and then you may proceed further to prepare the Accounts Report. 

Once the Accounts report is created, you may proceed to Tasks >> Submit >> Click on + Submit Accounts >> Choose the Accounts Report and then click on Submit to Companies House from the Accounts Production Module.

Please refer below screenshot for more help.

Click here to watch how to prepare dormant accounts