How to make a program from 0x0800 0400 that is on page 1 of stm32f042.

I want to leave page 0 of this microcontroller free to enter or not in the DFU boot.
I'm using the IROM1: modify feature to directly place the address I want, but to no avail.
Could anyone tell me what I need to configure in keil for this to work?

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