Help to, Change boot from internal memory to MicroSD

[Kenzox13]

Normally when you install a custom rom, the boot reads directly from the internal memory, not from the factory.
What's my problem?
I have batocera or my arcade IMG in the MicroSD, with the default OS, if I leave the MicroSD in the slot, it loads the content of the SD, if I have it outside, it loads the content of the internal memory.

But when I install any type of custom rom, no, it doesn't load from the MicroSD if I don't press the reset button.

If I'm not mistaken, I suppose I can somehow tell the custom room to read the MicroSD first, right?

Thank you