Step 1: Sub to this mod AND the "TheGathering sandbox mod"
Step 2: Start new game and choose the faction you want to play, in my case I chose Liu Chong
Step 3: At turn 1, open the sanbox mod's ui and insert this command "post.leader, liu_chong, king_sansang"
Step 4: Check you faction ui, the leader should now be Go Yeon-u. Now... insert this command "post.heir, liu_chong, prince_uwigeo" to change your heir
Step 5: Save and reload, your new ui should show Go Yeon-u as faction leader, the flag and the playstyles/resources will remain Liu Chong's though, but this pretty much let you start as Goguryeo.

IDs
Go Yeon-U = king_sansang
Go Uwigeo = prince_uwigeo
Lady U = lady_u
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v.2 edit;

Go Nammu = king_gogukcheon
Eul Paso = eulpaso

The 190 Expanded already included (King) Gogukcheon with unique Korean resources,
I would suggest you kill mine off if you're playing this mod with 190 Expanded, since mine use generic Han dynasty resources, nothing special. Or rename it, there's a mod Ironic did that do just that :)