If there is enough room, a PlexiDor can be installed under a window much like a regular wall installation.
The biggest issue with installing a pet door under a window is the limited space. Small and medium size doors usually fit, but if you want to put in a bigger door, the window might be in the way. Measure carefully and consult our size chart for wall installations.
Also consider how you want your security plate to work. The metal plate normally screws onto the frame, and that isn’t a problem under a window. If you want to use the sliding track system, the window sill might be in the way.