You have to recompile the firmware. See the file I linked to above.
Follow this guide:
Home · openmv/openmv Wiki · GitHub (make sure to clone recursively).
The make file TARGET is OPENMV4… e.g. make TARGET=OPENMV4
Firmware output will be in src/build.
Note, you may run into an issue related to MicroPython’s cross compiler. You will likely have to make it separately before the main firmware build. So, cd into src/micropython and follow the README in there to build their cross compiler.
Finally, update the file I linked to above with an alias (e.g. the name in MicroPython) and then the actual pin name (e.g. PB12 or something like that). You can get the actual pin name from the Eagle schematic. Note, the alias can just be the actual pin name.
Once this is done the Pin Alias can now be created as a Pin object like other pins and you can attach an interrupt to it and etc.
Use OpenMV IDE → Tools → Run Bootloader to load the firmware.bin file in the build output.
I can also compile the firmware specially for you and just add this. However, if you can go through the above it will save you time waiting on me.