In the editor:
Make sure that the Tilemap sidebar is opened.
(If not, go to the "VIew" tab on top of the program and make sure that Tilemap Bar is ticked)
You can view the Tilemap sidebar by clicking on the tilemap header on the right side.
Now, select your tilemap object, open the tilemap bar, click the tile you want to place, click the pen tool and click anywhere in your layout viewer on the tilemap object to place a tile.
In the event sheet:
Use the tilemap action "Set tile at".
For the tile ID, use the index of the tile you want to use.
(see the above mentioned tilemap bar for reference)
As long as the tilemap has the "platform" or "solid" behavior, you should be fine.