Deadbush on sand bock

There might be a tag for it. If not, you may be able to manually override the world feature to accept your custom block as a valid placement. (To be clear, dead bushes can be placed on the block by players, but you're trying to get it to naturally generate there?)

To override the world feature, you would need to add a 'minecraft' folder alongside your mod folder in your workspace, (workspace/src/main/data), and then copy the world feature data structure. (You can refer to MCAsset to get an idea of how this works. It'd be something like minecraft/data/minecraft/worldgen.) The basic idea is if you copy the file format and naming exactly, your mod will load first and override the vanilla world feature of the same name.

I'm pretty sure you would need to override either the configured feature or placed feature of the dead bush to get it to spawn on a modded block if it's hard coded and not a tag. (Not sure exactly where or which, but that's the gist of it.)

If you mean feature where deadbushes can potentially spawn on later, also make sure to use generation step that happens before vegetal decorations