GMS 2 Create Object on a (matching) tile

Hi - I am trying to create an object where a tile/tileset has been placed.

I have a background layer, which I am building with tilesets, which will not change or be interacted with. But I need an identical layer above it made up of objects.

Simplified, something like: If tileset at (x,y) = grass_tileset{instance_create_layer(x,y,"objects",o_grass);} etc....

I have tried various tileset_get... functions, but cannot get it to work.

Any ideas? Many thanks.