Hey there @faqu! Welcome back to the forums! It’s a bit odd to hear it’s not in the property window at all, this is somewhat a problem since most of the documentation requires at least one of the property windows to contain the checkbox for it. Nonetheless you can always check the generate checkbox in the static mesh editor. A bit of documentation so you can see what I mean is here: Generating Lightmap UVs in Unreal Engine | Unreal Engine 5.0 Documentation
Hope this helps!