Step 7.4. Override Data Tag/Data Client

1.  Add a Facebook Conversion API Tag to your server GTM container by clicking Add to Workspace.

2. Create a new tag and select the tag type Facebook Conversion API Tag. Select Inherit from client and add:

• FB API Access Token. 
• FB Pixel ID.
• Test ID. 
• Choose an event name from the standard events list or create a custom event name. For example, I chose the AddToCart event.
• Go to the Server Event Data Override section and choose "Event ID" for the Property Name. Then, in the Property Value field, add the {{event_id}} variable that was created in step 6.
• Inside User Data section and Custom Data you can add any user or product data you want to send to Meta. 

3. Configure trigger. In my case it’s a custom event with the event name add_to_cart_stape and client name contains DC.