function visit_crypt(hero)
	QuestionBox(GetMapDataPath() .. 'visit_crypt.txt', 'crypt_ok', 'crypt_cancel')
end

Trigger(4, 'crypt', 'visit_crypt')

function crypt_ok() -- this function is called when the player presses the OK button
	-- ...
end

function crypt_cancel() -- this function is called when the player presses the Cancel button
	-- ...
end

Added 15 minutes later
You can also create a function that will immediately return the player's answer, instead of calling another function. I think this is a bit more convenient.

function QuestionBoxRef(msg)
	question_box_ref_answer = -1
	QuestionBox(msg, "question_box_ref_answer=1 --", "question_box_ref_answer=0 --")
	while question_box_ref_answer == -1 do
		sleep()
	end
	return question_box_ref_answer == 1
end

-- example of usage

function visit_crypt(hero)
	if QuestionBoxRef(GetMapDataPath() .. 'visit_crypt.txt') then
		-- the player agrees
	else
		-- the player declines
	end
end

Trigger(4, 'crypt', 'visit_crypt')