You can get currently loaded levels from UWorld (UWorld::GetLevels).
You’d have to iterate through these, and somehow figure out which one is the one you want to spawn you actor on.
If you want to be able to have a dropdown list of levels in blueprint, you could use TAssetPtr. It’s important to not use simple reference as it would cause that any blueprint having such node, would load whole level to memory whenever that blueprint would be opened. TAssetPtrs prevent from doing this (among other things).
Also - which is even more important - selecting level in such dropdown, would actually provide reference to a Level ASSET. So you’d still have to iterate through loaded levels of current world and for example compare their names to find the requested one.