[LilBudyWizer]

Just set a variable while processing the addon loaded event. You can't avoid on /reloadui unless you want to avoid the initial load as well and never show the message. You can avoid displaying it after the initial player activation though.

local function onAddonLoaded()
    addon.firstActivation = true
end
 
local function onPlayerActivated()
    if (addon.firstActivation) then
        addon.firstActivation = false
        Print("Hello World!")
    end
end