I'm a DIY'er - not a pro - but here's what I did:

The wiring with the magnetic sensor is installed in the window frame. The magnet is counter sunk into the window -I used a fostner bit (this drills a flat bottom hole) to drill the hole for the magnet.

Are these breakage sensors or open/close (magnetic reed) sensors?

Depends on the type of window and window frame you are using.
Most magnets are surface mounted on the window and the "reed" switch, With the wires attached are mounted on the frame-

Although those are great if someone is going to "lift open" the window, window contacts are pretty useless. Most of the time it will be a "smash and grab", so motion detectors or "tempanic" glass break sensors that "hear" the glass breaking are far more effective and faster reacting.

Glass break detectors can also be set for a "constantly armed" state on the alarm panel, so they are armed even when home-protects you (and kids?) if someone should break in while you are in a different part of the house-

Just a few suggestions for you!

thanks everyone. I plan on putting in an elk security system that also doubles as an automation system as well. the window sensors will be for security but also to let me know if we forgot to close a window. Seems to happen all the time. I also have planned strategically placed glass breaks and motions. I just really don't want to see the window sensors. So I think I've come to the conclusion to use the small plug sensors and drill a hole on the side of the window frame. That should limit the exposure to moisture.