I've never put anything on new spark plugs for lawnmowers or cars ! If the threads are dirty, you can squirt some WD-40 on them and wipe them off real good but otherwise you just put the plug in as it is !
When replacing a spark plug on a lawnmower what do you use to coat the threads on the spark plug?
Never put anything on the threads of a spark plug.
If you lubricate it, you run the risk of overtightening.
If you put sealant there, you run the risk of having to drill the plug out at some stage in the future.
Clean and dry is the order of the day.