The TargetControlID property specifies the ID of the element that should be 'tooltipified'. It can be set declaratively on the page:
<telerik:RadToolTip ID="RadToolTip1" runat="server" TargetControlID="Button1" IsClientID="true">
...and in the code-behind:
By default, RadToolTip assumes a server ID. If the ID is client-side, as in the case where an element is pure HTML and not a server control, then the IsClientID property should be set to True.