How can I set the second layer of an NPC visible using PacketPlayOutEntityMetadata in 1_18_R1 (without Mojang Mapping)?

Question

I'm trying to code a Minecraft Spigot Plugin using NMS. I want to create and spawn an NPC. The code I wrote for creating the EntityPlayer, GameProfile and for sending the packets works fine, but I can't figure out how to set the EntityPlayer metadata in 1.18.1. I've tried doing the same thing in 1.15 and it worked perfectly.

md_5 remapped a lot of methods. For example, to get the DataWatcher of an EntityPlayer, I used to do npc.getDataWatcher();, but now I have to use npc.ai();. I can't seem to find a solution on the internet that works in 1.18.1 without any other APIs such as Citizens.

I think I figured out all the methods counterparts, but it still doesn't work.

So, how did the way you set Metadata of an EntityPlayer change in 1.18.1?