There were two ways. One has recently stopped working. I asked why a few weeks ago, but didnt get an answer…
Currently, you’ll need to drop a running line around the room, everywhere you want baseboard. Then, use the add trim button on the items tab to select a baseboard molding, either in a generic catalog, or one that is offered by one of your cabinet vendors. Drop the molding onto the running line, and 2020 will extend it to all contiguous sections of the line.
You should check that it added to the correct height. Make a rendering, or look at an elevation. If it isnt right, you should be able to drag it down to where you want it.
This technique works with chair rail and crown molding, too. You can do quick wainscot this way, too. Just place a generic baseboard, resize it, and change the texture.