点击 vgui 按钮时如何调用函数

（我对 Lua 还比较陌生，请原谅我在解释时可能犯的错误）我正试图制作一个任务系统，以便玩家可以在打开实体并选择开始任务时点击任务。我似乎无法想出如何让我的“function npcquestkill()”仅在玩家点击开始任务按钮时启动。任务按钮是“local Button = vgui.Create("DButton", Frame)”。我该如何处理这个问题。

net.Receive("OBJ_OPEN", function()
        local Frame = vgui.Create("DFrame")
        Frame:SetPos( 700, 400 )
        Frame:SetSize( 500, 350 )
        Frame:SetTitle( "Objectives" )
        Frame:SetVisible( true )
        Frame:SetDraggable( false )
        Frame:ShowCloseButton( true )
        Frame:MakePopup()
        Frame.Paint = function( self, w, h )
            draw.RoundedBox( 0, 0, 0, w, h, Color( 231, 76, 60, 150 ) )
        end

        local Button = vgui.Create("DButton", Frame)
        Button:SetText( "Kill 10 Covenant in One Session" )
        Button:SetTextColor( Color(255,255,255) )
        Button:SetPos( 5, 30 )
        Button:SetSize( 490, 50 )
        Button.Paint = function( self, w, h )
            draw.RoundedBox( 0, 0, 0, w, h, Color( 41, 128, 185, 250 ) ) -- 绘制蓝色按钮
        end

        Button.DoClick = function()
            Button:SetText( "Kill 10 Covenant in One Session: In Progress")
            vbutton = 2
        end

        if (vbutton == 2) then
            Button:SetText( "Kill 10 Covenant in One Session: In Progress")
        elseif (vbutton == 1) then
            Button:SetText( "Kill 10 Covenant in One Session" )
        end
end)

function npcquestkill()
    npckills = npckills + 1
    print(npckills)
end

function rewards(ply)
    if(npckills == 50)then
        ply:AddMoney(1000)
        vbutton = 1
    end
end
hook.Add("OnNPCKilled","npcquestkill", npcquestkill)