If you use a styptic agent or a anti-hemorrage syringe on your medkit you can use the action button (whatever drops pallets probably? It's space on PC) to instantly heal.
Styptic Agent - Purple Add-on - Reduces charges by 25% and can be used to instantly heal 1 health state (injured to healed, downed to injured) as long as there is at least 1 charge left.
Anti-Hemorrhage Syringe - Pink Add-on - Reduces charges by 50% and can be used to instantly heal 2 health states (downed to full health or injured to healed, borrowed time bleedout to full heal as well) as long as there is at least 1 charge left.
Other tips for y'all, the add-on order matters!
Brown Medkit - use a styptic then + charges add-on and not the other way around
Base heal = 16 x 0.75 for 12 charges, then + 6 charges = 18 charges final. Enough for 1 full heal and a quick instant heal.
If you do the add-ons where the instant heal is 2nd it will decrease the charges for the 2nd add-on
16+6=22 x 0.75 for 16.5 charges. Could almost work for an instant heal, esp if you hit a great during the heal but you probably won't have enough time to hit the action button before it just disappears