Just put it in a sink of hot soapy water and use a sponge or scourer and leave it to dry. Clean the slot as well you'd be amazed at the muck that gathers in there
I dare you to shine a torch in to the drawer cavity and have a good look at the top and right at the back. You’ll be getting the scorer out, I guarantee it
I never use the powder drawer as I just sling what I need into the drum with the clothes, but the conditioner bit gets awfully blocked up and the unit won't fit in the sink, it's too long. It's an absolute burger to clean
I only put fabric conditioner, limescale tabs and vanish powder in the drawer but the yucky stuff still builds up after a while. I use cheap toothbrushes to clean the bits you can't reach with a sponge.
A washing machine man told me years ago all this happens because of cold water fills. He told me every so often pour a kettle full of boiling water in the drawer. I have done this ever since and it keeps it free of muck build up.
You can buy washing machine cleaner in supermarkets for a couple of pounds. You mix a little with water and soak the tray in the solution, after cleaning the tray you put the rest of the bottle into the tray and turn on a normal washing cycle, its great and also leaves your machine smellin nice and fresh. I've just looked in my cupboard and found two bottles, one is made by Dettol and one is Sainsburys own brand.
You can buy special washing machine cleaner in supermarket, it cleans the tray and inside machine. I have two at the moment, both bought from Sainsburys, one made by Dettol and one own brand. You dilute a little in in water and soak and clean the tray, then replace the tray and put the rest of the cleaner in the tray, then choose a washing cycle with empty drum and Bobs your uncle. They are really good.