HearToLearn:

We need a bit more info.
I'm not sure what you mean by "snap to key".
In the BIAB Piano Roll window, you can set the "snap" value setting in the top LH corner of the window, that can be anywhere from "none" to a 32nd note triplet.

That allows you to move the notes (or enter new notes) onto the grid, and they will snap to the horizontal position of your snap value setting.

If you drag an existing note vertically, you can move it to any note you wish, but it won't "snap to key", if you mean will it stop at a vertical note position that is in the correct "key" that your song is in.

Give us a bit more detail and perhaps we can help.
Thanks!
LLOYD S