The Mouse is inserted into Joystick Port 0.
This port shares pin 9 with pin 6 of Joystick Port 1 (fire button).
The ST-series joystick ports are compatible with Atari 2600 and Atari 7800
Paddles are not supported, though and unlike the XL/XE, the ST can not control the outputs of the first 4 port pins.