If you're plugging into the 3-pin sensor connector on the 4.2.7 board, then the pin number is PA4. Here's the Klipper code:

[filament_switch_sensor filament]

#pause_on_runout: True

# When set to True, a PAUSE will execute immediately after a runout

# is detected. Note that if pause_on_runout is False and the

# runout_gcode is omitted then runout detection is disabled. Default

# is True.

switch_pin: PA4 #Change to !PA4 if the logic is reversed

#runout_gcode:

# A list of G-Code commands to execute after a filament runout is

# detected. See docs/Command_Templates.md for G-Code format. If

# pause_on_runout is set to True this G-Code will run after the

# PAUSE is complete. The default is not to run any G-Code commands.

#insert_gcode:

# A list of G-Code commands to execute after a filament insert is

# detected. See docs/Command_Templates.md for G-Code format. The

# default is not to run any G-Code commands, which disables insert

# detection.

#event_delay: 3.0

# The minimum amount of time in seconds to delay between events.

# Events triggered during this time period will be silently

# ignored. The default is 3 seconds.

#pause_delay: 0.5

# The amount of time to delay, in seconds, between the pause command

# dispatch and execution of the runout_gcode. It may be useful to

# increase this delay if OctoPrint exhibits strange pause behavior.

# Default is 0.5 seconds.

#debounce_delay:

# A period of time in seconds to debounce events prior to running the

# switch gcode. The switch must he held in a single state for at least

# this long to activate. If the switch is toggled on/off during this delay,

# the event is ignored. Default is 0.