Click "Create Destination" in top right
Click "Authorize a new Facebook Custom Audiences acccount"
Complete the OAuth flow with Facebook
After being redirected back to Hightouch, select an Ad Account using the Dropdown and then click "Save"
All that you have to do to connect a query to Facebook Custom Audiences is setup mappings between the columns returned by your SQL query and Facebook.
The columns on the left hand side are from the SQL query and the columns on the right hand side are from the Facebook Ads
The mappings supported by Facebook are pretty standard with a few exceptions, e.g.
External ID is a custom identifier that you send to Facebook via its APIs or tracking pixels https://developers.facebook.com/docs/marketing-api/conversions-api/parameters/external-id/
Page-scoped ID is used for Facebook messenger apps and Facebook pages