display JS message on tab control load

I have an interesting dilema. I have a user control which is being added through tab control with 2 other controls. I want to display a popup message upon user control click through tab control.

I am wondering how I can do it using mainpage.aspx, once abc.ascx control is activated.

P.S. I have tried simple popup message on abc.ascx control and it doesn't have following method. I am stuck Please help!!!

ScriptManager.RegisterStartupScript()

ScriptManager.StartupScript()

this.gettype()