Paint the level in the tilemap in Construct then load your game and trigger the tilemap Download action during the game to save a json file. You can then store the json string from the file somewhere in your game like in a variable or array and use the Load action to load it later. The downloaded json string contains all the data for the level you drew with the positions and type.
I think you could also set a string variable to tilemap.tilesjson on start of layout which is the same data as above and then pick it out in debug preview, you can click on the variable value field and copy that data to save it for later.