As an action, you can spend one use of a healer’s kit to tend to a creature and restore 1d6 + 4 hit points to it, plus additional hit points equal to the creature’s maximum number of Hit Dice. The creature can’t regain hit points from this feat again until it finishes a short or long rest.
Nowhere does it mention that the target needs to be immobile or that this has to be done outside of combat. D&D is not a realism simulator and it doesn't really matter that realistically,you wouldn't be able to bandage a person while they are dodging and weaving through a combat scenario. Realistically, you could also bandage somebody more than once if they have multiple wounds.
For gameplay balance, it has been decided that you can use the action at any point, but only once per short/long rest. Houseruling that you can only do it outside of combat while they are immobile is a perfectly valid houserule, but it'll ensure that nobody in their right mind will take the feat as it turns it from a pretty good healing feat to a very mediocre feat at best.