Can I make my interior paint rainproof?
Once fully dry, your paint should be quite waterproof. However paint designed for interior use on wood generally will not hold up well when used outside on plastic. 

Probably the cheapest way to go would be a can of spray-paint and some masking tape. Use the tape to outline a square or rectangle. Use more tape to hold down some old newspaper, cardboard, whatever to keep overspray from making a mess. Apply several light coats of paint in accordance with the manufacturer's directions. Allow each coat to dry per the instructions in a dry location.

Once it's dry, use a marker of a contrasting color to write your house number on the paint.

Alternatively, you could use that tape to outline some numbers then mask off the rest of the bin and directly paint the numbers.

Another alternative would be to use a marker (in a color that would be visible against the color of the bin) to just write directly on the bin. This is only applicable if it meets the regulation that caused you to start the project in the first place.