Much depends on where you live. There’s a big difference between conditions in Los Angeles and Buffalo.
Basically there are three approaches.
Buy a purpose built device that doesn’t integrate with SmartThings. Lots exist.
In relatively mild areas you can just shelter a regular contact sensor and hope for the best. It may only last a year or so, but might fit your needs. Some people put it inside a plastic box.
In severe environments, you solve this with a two piece set up. You get a weatherproof contact sensor (not made for SmartThings) and wire it with a long wire to a SmartThings-compatible contact sensor that is sheltered from the weather.
