Crop & Export 1:1 Ratio (Export Settings > Source > Crop)

This is a question that comes up every so often, but it is possible to do, there's just not a preset. This feature hasn't been touched in a long time, and it was around long before 1:1 video was popular. You can always feature request it at the UserVoice site: https://adobe-video.uservoice.com/forums/911233-premiere-pro

All you have to do is change the "Crop Proportions" dropdown menu to "None" and set your own Left and Right px values. For example, if you import a 1920x1080 video and crop 420 pixels from both the left and right, then you'll end up with a 1:1 crop and you can export a 1080x1080 video.