Can you post a picture of the door with the surrounding wall and door frame? There are some possibilities, but it depends very much on the exact architecture of the door.
The first issue, and this is important, is it generally not a good idea to post a surface mount magnet piece below about 3 1/2 feet off the floor. That’s because if either a child or a dog manage to get the magnet piece off the door and swallow it it’s a very serious medical issue. 
There might also be issues with the metal frame, but if it’s a patio door, these are more commonly aluminum, which doesn’t present issues. But if it’s a steel frame, that is a problem. (You can tell whether the doorframe is ferrous just by taking a regular kitchen magnet and seeing if it sticks to it. If it sticks, then there are going to be issues with using magnetic sensors.)
So the best approach is to use a recessed sensor or microswitch in the wall such that when the latch closes it makes contact. A couple of community members have done this for some exterior doors, but I don’t think anyone’s done it for a sliding door.
Another alternative is to put the micro switch inside the turn bolt, but I think that most of the ones for patio doors would be too small. But again it depends on the specific door you are working with.
So it all depends on the wall around the door frame as to whether that’s doable or not. And on the material of the door frame itself.
Here are some project reports that present the general idea:
Here’s one with the switch inside the turn bolt casing. Note that that’s a very old thread and discusses the “thingshield” which is no longer made. But there are other options available now (2019) which would accomplish the same thing.