Go to home improvement store or hardware store and ask for a cap for a under sink line and show them this picture. The user access only has 2-3 sizes and this is by far the most common for water.

Get good Teflon or equivalent, wrap opposite direction as the threading so counter clockwise (so you don't unravel as you turn) 2 wraps is sufficient. Cut 90° and lay flat as wrinkles will lead to leaks. More isn't better

Obviously turn valve off as well and if you see any calcium hit it with clr type cleaner now while you have access

Cap hand tight and check for leaks. Now you have access to a water line when you need it down the road