[Garkin]

If you want to store counter, you have to use saved variables - i.e. move your counter to saved variables table.

Just to show something different, I have also moved your code from XML to LUA.

local QuestCounter
local SV
local defaults = {
    Count = {
        offsetx = 0,
        offsety = 0,
        point = CENTER,
        relPoint = CENTER,
    },
    ClickedCount = 0,
}
     
local function ClickCount()
    SV.ClickedCount = SV.ClickedCount + 1
    QuestCounter.Count:SetText("Completed: " .. SV.ClickedCount)
end
     
local function ResetCount()
    SV.ClickedCount = 0
    QuestCounter.Count:SetText("QuestCounter")
end
 
local function OnMoveStop(self)
    local valid, point, _, relPoint, offsetx, offsety = self:GetAnchor(0)
    if valid then
        SV.Count.point = point
        SV.Count.relPoint = relPoint
        SV.Count.offsetx = offsetx
        SV.Count.offsety = offsety
    end
end
 
local function CreateAddonWindow()
    --main window
    QuestCounter = WINDOW_MANAGER:CreateTopLevelWindow()
    QuestCounter:SetAnchor(SV.Count.point, nil, SV.Count.relPoint, SV.Count.offsetx, SV.Count.offsety)
    QuestCounter:SetClampedToScreen(true)
    QuestCounter:SetResizeToFitDescendents(true)
    QuestCounter:SetMouseEnabled(true)
    QuestCounter:SetMovable(true)
    QuestCounter:SetHidden(false)
           
    --label
    QuestCounter.Count = WINDOW_MANAGER:CreateControl(nil, QuestCounter, CT_LABEL)
    QuestCounter.Count:SetAnchor(CENTER, QuestCounter, CENTER, 0, 0)
    QuestCounter.Count:SetFont("ZoFontBookScroll")
    QuestCounter.Count:SetColor(ZO_NORMAL_TEXT:UnpackRGBA())
    QuestCounter.Count:SetWrapMode(TEXT_WRAP_MODE_ELLIPSIS)
    QuestCounter.Count:SetVerticalAlignment(TEXT_ALIGN_CENTER)
    QuestCounter.Count:SetHorizontalAlignment(TEXT_ALIGN_CENTER)
    QuestCounter.Count:SetText("QuestCounter")
end
     
local function OnAddOnLoaded(eventCode, addon)
    if addon == "QuestCounter" then
        --this event is no longer needed, unregister it
        EVENT_MANAGER:UnregisterForEvent("QuestCounter", EVENT_ADD_ON_LOADED)
 
        --initialize saved variables
        SV = ZO_SavedVars:New("Cou_Variables", 1, defaults)
 
        --this will create the same window as in your XML. There is just one difference - I made controls unnamed (there is no global reference to UI elements)
        CreateAddonWindow()
 
        --mouse click handlers
        QuestCounter:SetHandler("OnMoveStop", OnMoveStop)
        QuestCounter:SetHandler("OnMouseDoubleClick", ClickCount)
        QuestCounter:SetHandler("OnMouseWheel", ResetCount)
 
        --slash command
        SLASH_COMMANDS["/qlc"] = function()
            QuestCounter:ToggleHidden()
        end
    end
end
     
EVENT_MANAGER:RegisterForEvent("QuestCounter", EVENT_ADD_ON_LOADED, OnAddOnLoaded)