Will I test positive on a pre-employment test having taken finasteride?

Tests are specific. Your company will instruct the doctor/technician to test for drugs or alcohol or marijuana.

If they're testing for marijuana, it wouldn't matter if you were drug as a fish.

I had a friend who consumed marijuana (in a cake) and admitted it to the company doctor doing the blood test. He informed her, it didn't matter, because the testing was for something else.

Finasteride will show up in your urine/blood.

Most things do.

But in any event Finasteride is not illegal.