With your inventory open, you can normally press the offhand swap key above an item to swap it with your offhand. This doesn't work with Multiconnect 1.4.2 on Minecraft version 1.17. I downloaded Multiconnect from the Releases page. I also tested this with no mods installed besides Multiconnect and the behavior stays the same, works when I remove Multiconnect.

Interestingly, all offhand swaps trigger after I close my inventory (not while it's open).