STM32F411VEtx enabling DFU mode from Application code

Hello All,
Am trying to make the STM32f411VEtx board into DFU mode from the Application code, so am trying to write the bootloader code, If any one is having code please share or please help me how to write the code and how to flash as well.
i took a reference code from git and tried to adopt the code for STM32f4 but unable to flash the code via ST flash loader tool

Thanks

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