Problem custom structure custom mob spawning

Hello everybody, I would be so grateful if somebody helps me. I'm trying to make a mod that contains custom mobs, that spawn in custom structures. I saved the structure with the structure block, and allowed to save the mobs too, but my custom mobs weren't saved. and I don't want to put a spawner because it would ruin my structure, not just how it looks but the idea of the structure.

I tried to put an procedure, to make the mobs spawn, but I didn't work either, any ideas?

I'm very grateful if you leave your idea