Button1.Attributes.Add( "onclick" ,  "javascript: alert('click');" );

Button1.Attributes.Add( "onclick" ,  "click()" );
string script =  "<script language='JavaScript'>\n" ;
script +=  "<!--\n";
 
script += "function click() {alert('click');} ";
script += "  -->\n";
 
script +=  "</script>\n" ;
if (!Page.IsStartupScriptRegistered( "ScriptHandle" )) 
	Page.RegisterStartupScript( "ScriptHandle" , script);