Easiest way is to learn how to do a bunny burrito – it’s basically wrapping your bun up in a towel or small blanket so they can’t wriggle, with only their head out for the medication.
Then just syringe it in his mouth. It’s the best way to be sure he gets it all – he may not eat it on the lettuce, or it may get smeared on other things, and be wasted.
If it’s the stuff I am thinking of many buns do like it, but that doesn’t mean he will let you just give it to him. I had a rabbit on some medication that was really sweet and secretly she loved it – it never got wasted. But she always had to protest it first on principle – hence the bunny burrito so it is far less likely to be wasted.
Bunny burrito also saves you from getting scratched!
If you can have a second person on standby at least the first time. That way one of you can hold your burrito bunny and the other can do the syringing. 