I want the URL to contain the DM reply message body

URL: https://domain.com/?source_message_body={{message.body}}

So if the contact sends 'hello' as a message, the link being sent to them in the reply should be 'https://google.com/hello' for example (This is to be used for tracking purposes)

But it doesn't work - it doesn't map the message body in there at all, it's just empty.

Most likely this is a bug that needs to be fixed where the IG interactive DM reply button URL isn't able to access the mappable fields, so it just returns them as empty.

The button URL does allow mapping contact fields and custom fields; it just doesn't allow mapping trigger action fields (like this example ).