Use feverfew mixed with peppermint in a tincture applied to the skin (use a vodka base, not rum/whiskey).
As someone with tons of experience with herbal tinctures, I would STRONGLY recommend against using Pennyroyal on dogs or cats in any capacity.
For one, it isn't even that great of a flea/tick/mosquito deterrent compared to other herbs.
More importantly, Pennyroyal, when ingested, has toxic qualities; hence why it is used as an herbal abortive, and can cause internal bleeding (and eventual death) when taken in a large enough doses or small residual doses over time.
Both dogs and cats, ingest most solvents and debris from their fur when they clean themselves to lick; which means you would be slowly poisoning your dog over time.