In update 17.2, which will be released in the main client of PUBG: Battlegrounds next week, the developers have added a new feature to the game that allows you to use bandages and boosts on the driver’s seat in vehicles (including boats). This applies to the following items:
Bandages Energy Drinks Painkillers Adrenaline Syringe
First aid kits still cannot be used. Any driver action other than moving forward will cancel the use of items.