If the first bet unlocks the free bet whether it wins or loses, you should make the first bet on short odds (because you unlock the free bet regardless of outcome, and bets with shorter odds have lower variance and lower expected tax liability) but make the free bet on long odds (because the EV is higher the longer the odds for free bets that do not return their stake).
In the case that you have to lose your first bet to unlock the free bet, but the free bet returns the stake, the first bet should be made using long odds (to allow you to unlock the free bet as often as possible) but the free bet should be made using short odds (because bets that return the stake have the same EV regardless of odds, and shorter odds helps lower variance and expected tax liability.)
I should also add: for risk-free first bet promos where the free bet returns the stake like Fanduel and Barstool, your first real-money bet should be at very long odds because a free bet that returns the stake is as good as cash (providing even more incentive to unlock the free bet as often as possible.)
Great clarification. What I meant to say was:
If the first bet unlocks the free bet whether it wins or loses, you should make the first bet on short odds (because you unlock the free bet regardless of outcome, and bets with shorter odds have lower variance and lower expected tax liability) but make the free bet on long odds (because the EV is higher the longer the odds for free bets that do not return their stake).
In the case that you have to lose your first bet to unlock the free bet, but the free bet returns the stake, the first bet should be made using long odds (to allow you to unlock the free bet as often as possible) but the free bet should be made using short odds (because bets that return the stake have the same EV regardless of odds, and shorter odds helps lower variance and expected tax liability.)
I should also add: for risk-free first bet promos where the free bet returns the stake like Fanduel and Barstool, your first real-money bet should be at very long odds because a free bet that returns the stake is as good as cash (providing even more incentive to unlock the free bet as often as possible.)