There are a couple of options.
Firstly, if you check, you will find that it is illegal to smoke airside at many airports. This includes on the aircraft. You could perhaps arrange a "spot check" to happen. Or you could mention this, in writing, to the company and ask if you are allowed to smoke or not.
Second, if the Captain wants to smoke, tell him that that is fine, but you will leave the flight deck for the duration.
Third, take up a foul smelling habit yourself. One good plan is to get a really strong smelling deodorant or aftershave and spray it about liberally either during or after the smoking.
This is a health and safety issue. You should perhaps query it again with the company, in writing, and ask them where you stand.
The company I work for used to allow smoking on the flight deck. It was then banned. And everyone managed to get through even long haul sectors without a cigarette.
Now the company I work for has banned smoking airside too, after one crew had a "spot check" and received a warning as it was illegal.
Some people go off to find a smoking area during turnround, but most don't. Some have taken the opportunity to reduce the amount they smoke or try to give up altogether. Some use patches. But most just cope with it.
Perhaps if someone is so addicted to a drug that they cannot function without it, then they shouldn't be flying?