Outgoing port for backups?

I have 2 internet connections. I’d like to force duplicati to use 1 internet over the other. I need to do this policy based routing inside my router.
I can do it based on source port so I’m wondering what outgoing port duplicati would use to connect to google drive for the backups?

This way, if my router sees a connection from [internal server ip] on port [duplicati port] going out to the internet it can route it across the internet connection I want.

Duplicati is running on a windows server 2019 box using google drive as the backup destination.

Using multiple ethernet connections? (probably a followup to this topic) wound up with the discussion.