I am working on creating a custom biome, and I am only running into a single issue. Vanilla mobs will not spawn on my custom block that is the surface level block. Is there any way to make mobs spawn on a custom block?

What have you tried? 

I've tried having the block extends GrassBlock and I've tired adding
@Override
public boolean canCreatureSpawn(BlockState state, IBlockReader world, BlockPos pos, EntitySpawnPlacementRegistry.PlacementType type, @Nullable EntityType<?> entityType)
{
return true;
}
@Override
public boolean canEntitySpawn(BlockState state, IBlockReader worldIn, BlockPos pos, EntityType<?> type)
{
return true;
}

the second is deprecated don't use it

and I guess you have to add the entities that you want to spawn somehow to your biome 

I already have entities spawning in my biome.
this.addSpawn(EntityClassification.CREATURE, new SpawnListEntry(EntityType.CHICKEN, 50, 1, 10));

this.addSpawn(EntityClassification.CREATURE, new SpawnListEntry(EntityType.COW, 50, 1, 10));

this.addSpawn(EntityClassification.CREATURE, new SpawnListEntry(EntityType.PIG, 50, 1, 10));

this.addSpawn(EntityClassification.CREATURE, new SpawnListEntry(EntityType.SHEEP, 50, 1, 10));
and when I set the surface block to generate as grass they spawn, but when it's my block they don't.