My company trades under a different name to its registered name: how do I show the trading name in Engager?

Change the Company name field in TaxCalc to your trading name. That's the field that syncs with Engager (the sync works both ways), where a Limited Company has a single Name field and no separate trading name field.

Keep your registered company name on file by storing it in a custom field in Engager (Settings > Client data > Custom fields) for reference.

Sole traders are different: their record has its own Trading name field in Step 1: General info.