DFU ROM bootloader, external pull-up on D+ via GPIO, which GPIO does DFU ROM want?

[crackwitz]

i have an STM32F303CB/CC, self-powered, doing USB. appnotes AN2606 and AN4879 apply. "A DP pull-up should be connected only when VBUS is plugged, a GPIO from the MCU is used to drive it after the VBUS detection."

nowhere is mentioned that the DFU ROM bootloader needs a specific GPIO to serve this purpose. it needs to engage the pull-up though. also I don't see a specific GPIO used for VBUS sensing...

what do I need for the DFU ROM bootloader to work?