There's no slot to drop them into except the usual to create a new regiment when a hero is next to the town. While no hero is in town I can only see garrison and army. When I switch there's nothing. Only when I get conscripts can I right click them to remove them.

EDIT: Correction, it only borks and doesn't work when the UI scale is too high. Except I need to get the scale below 100% to even see the option to. I normally play on 141% because that's what auto-scale does and it looks the best. Honestly I think the UI doesn't properly scale. If the UI scaling was reworked to work on a standard laptop wide-screen resolution then it would be much better. As it is I had to also reduce the scale so that I could read the message that says I can change the town for 5k gold and 10 wood and ore. If I didn't scale down the UI I would never be able to read this and I had to squint to read the small letters anyway.