Applying Damage

You can right click to deal damage or healing to a selected token.

Hatmer in this situation is attacking the Toad-Goyle. He hits for 14 damage. I have selected the Toad-Goyle and right clicked on the attack chat message - you can see the "Apply Damage" option.

After choosing this option, the 13 damage was applied to the Toad-Goyle token's HP, and you can see a messaging confirming that in the chat.

You can do the same thing after a cleric rolls Lay on Hands, but choose Apply Healing.