The configuration for the external flash is contained in
stm32cubeide_1.3.0/plugin/com.st.stm32cube.ide.mcu.debug.openocd_1.3.0.202002181050/resources/openocd/st_scripts/target/stm32f7x.cfg
A quick'n dirty hack would be to adapt the existing configuration file to your board's pin usage (i.e. 'qspi_init' proc) but still retaining the board's name as STM32F769I-DISCO or inventing a new board name and add the appropriate setup to this file.
The clean solution is creating a completely new configution file using the one above as a template only. However, you would have to figure out how to use the new one instead of the original one, this I don't know unfortunately.
