First of all you need to be in a guild
When your in a guild there is a small arrow on the left hand side of the screen click on the arrow that will bring up the chat you can only send heals if people request them but to send them a message will come up saying : who the person is that has requested the heals then it will read I need some heal kits please and you literally just click on the person and then you've just sent a heal.
Hope I helped you
enjoy the game