How To Execute Server Side Code From This Button

I have a button where I use an Animation Extender. When I click the button I have to set OnClientClick="return false;" otherwise the Animation Extender closes.
So that causes my problem. How do I execute server side code from this button?
I have tried this code in my code behind, but there is an error:

Code:
Dim strScript As String
strScript = "<script>"
strScript = strScript & "return false;"
strScript = strScript & "</script>"
ScriptManager.RegisterStartupScript(Me, GetType(Page), "MyScript", strScript, False)
This is my code for the Animation Extender:
Open data"></asp:Button>