As far as I know you can just set the extra_tags in the quest, and then reference that same custom tag in the QuestTags in the POI's xml.
Odd you mentioned that, I have tried to set them in the quests.xml but it doesn't seem to work, unless I'm being stupid and placing it wrong.
The Quest Property XML (obviously there is more like objectives but I don't believe relevant?)
<quest>
<property name="name_key" value="quest_TSIQ_Friend1"/>
<property name="subtitle_key" value="quest_TSIQ_Friend1_subtitle"/>
<property name="description_key" value="quest_TSIQ_Friend1_description"/>
<property name="icon" value="ui_game_symbol_quest"/>
<property name="repeatable" value="true"/>
<property name="category_key" value="quest"/>
<property name="difficulty" value="medium"/>
<property name="difficulty_tier" value="2" param1="difficulty"/>
<property name="offer_key" value="quest_TSIQ_Friend1_offer"/>
<property name="statement_key" value="quest_TSIQ_Friend1_statement" />
<property name="response_key" value="quest_TSIQ_Friend1_response" />
<property name="login_rally_reset" value="true"/>
<property name="completiontype" value="TurnIn"/>
<property name="completion_key" value="quest_TSIQ_Friend1_completion"/>
<property name="shareable" value="true"/>
<property name="extra_tags" value="redhuntfriend"/>
</quest>
The POI XML:
<?xml version="1.0" encoding="UTF-8"?>
<prefab>
<property name="CopyAirBlocks" value="True" />
<property name="ExcludeDistantPOIMesh" value="False" />
<property name="ExcludePOICulling" value="False" />
<property name="DistantPOIYOffset" value="0" />
<property name="AllowTopSoilDecorations" value="False" />
<property name="DifficultyTier" value="2" />
<property name="ShowQuestClearCount" value="1" />
<property name="TraderArea" value="False" />
<property name="PrefabSize" value="45, 16, 26" />
<property name="RotationToFaceNorth" value="2" />
<property name="Zoning" value="none" />
<property name="YOffset" value="-5" />
<property name="EditorGroups" value="rural, house old" />
<property name="Tags" value="town,residential,countrytown,countryresidential,rural,rural_filler,ruralfiller,nodiagonal,culdesac" />
<property name="QuestTags" value="clear, fetch, redhuntfriend" />
<property name="SleeperVolumeSize" value="14, 6, 19#20, 3, 18#20, 3, 20#25, 2, 21" />
<property name="SleeperVolumeStart" value="3, 5, 4#21, 1, 5#21, 5, 3#18, 13, 3" />
<property name="SleeperVolumeGroupId" value="0,1,2,3" />
<property name="SleeperVolumeGroup" value="S_-Group_Generic_Zombie,4,24,S_-Group_Generic_Zombie,5,11,S_-Group_Generic_Zombie,3,16,S_-Group_Generic_Zombie,6,17" />
<property name="SleeperIsLootVolume" value="True,False,True,True" />
<property name="SleeperIsQuestExclude" value="False,False,False,False" />
<property name="SleeperVolumeFlags" value="0,0,0,0" />
<property name="SleeperVolumeTriggeredBy" value=" # # # " />
</prefab>
Am I genuinely putting something in the place?