This article will show to GameObject Brush from the 2D Tilemap Extras. The GameObject Brush is used to paint GameObjects onto the Tilemap instead of placing them manually.
The GameObject you want to paint needs to be a child of that Tilemap, and the copies that get painted also become children of that Tilemap. So to keep it organized, you can create a new Tilemap for the GameObject.
In this case, we are going to use a Prefab as the child GameObject. Align the GameObject to the Tilemap Grid for easy selection and painting.
With the correct Tilemap set as the Active Tilemap, change the Brush in the lower-left dropdown menu in the Tile Palette window from Default Brush to GameObject Brush. You can now use the Picker Tool to select the grid square(s) with the GameObject and begin painting with it.