[sticklord]

Im pretty sure it's impossible to hook the mixin, so you need to hook the frame. Maybe this will work (untested)

local function ShowBountyTooltip(self, bountyIndex)
    print(bountyIndex)
end
 
for _,frame in pairs({WorldMapFrame:GetChildren()}) do
    if frame.BountyName then
        hooksecurefunc(frame, "ShowBountyTooltip", ShowBountyTooltip)
    end
end