There is no voucher or code. Apple thought of a way where such isn’t needed. The redeem procedure does depend on the account however, and that the account didn’t have Apple TV+ before.
Read here how to redeem: Apple TV+ offer - Apple
Read here about the conditions: Apple TV+ offer - Terms and conditions - Apple
Read here if you have trouble redeeming: If you can't redeem your promotion for an Apple subscription - Apple Support
My iPad is listed on my husbands account
Apple ID accounts are used for individual settings, and it is recommended that each user has their own account. There is a Family Sharing feature available to combine services for a small group of accounts. But this doesn’t apply to original question (anymore), as the offer is for the first registered account on the new device.