btn <number>,<number>,<number>,<number>,<number>,<number>,<number>

From left to right, this is: button #, x, y, width, height, x3, y3.
This is the command you use to create and initialize buttons. Button numbers start from 1.
The resultant button starts at position (x,y) and is (width,height) big.
When the mouse cursor comes over the button, it will be displayed using a rectangle of the same size from the btndef bitmap with top left at (x3, y3).