You don't need an event for this since by default, the end user cannot add TabPages to the TabControl without you providing the code for it.
So wherever you are adding the TabPage, that's when you should wiring up those events:
TCTabPage tctab = new TCTabPage();
tbtab.Text = "New Tab";
tctab.MouseEnter += tctab_MouseEnter;
tctab.MouseLeave += tctab_MouseLeave;
customerTabCtrl.TabPages.Add(tctab);