How do I generate custom trees in a Nether-Like Dimension?

In Minecraft make the custom tree(s), save them with a structure block, import that *.nbt to MCreator. https://www.youtube.com/results?search_query=structure+block+tutorial+java+1.16

How to make a biome? - https://mcreator.net/wiki/how-make-biome

Add structure spawn (one for each), select the structure recently created, Spawn world type: Nether, Restrict to biome: select a pre-existing nether biome.

Build the tree in minecraft then save it as a .nbt file with structure block and add that .nbt file as a new structure spawn  then restrict it the to biome u made and set spawn world type to the dimension u made

You may not use custom trees from your biome in a nether type dimension, as they will not spawn. You need to create a structure, as the previous comments suggested, and you need to make the structures spawn in your custom biome, you also need to set it so that it will spawn underground (everywhere in nether like gen is classified as underground). You Need to make custom spawning conditions with a procedure so it will not be floating in the air, So far I have had trouble getting the trees to work properly, I currently have it so If the block below the structure is my dirt block of my biome, and the surrounding area of the base of my tree is air, then it can spawn. But this has caused it so very few trees spawn, usually 1 per chunk. This happens even though I have it set to the structures will spawn in groups, if anyone knows how to properly use trees in nether like gen, please let me know as it would be very helpful.