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Page page = HttpContext.Current.CurrentHandler as Page;

String jscript = "window.open('https://google.com', '_blank')";

ClientScriptManager csm = page.ClientScript;
 
csm.RegisterStartupScript(page.GetType(), "salesforcescript", jscript,true);

Page page = HttpContext.Current.CurrentHandler as Page;

String jscript = "window.open('https://google.com', '_blank')";

Control control = page.FindControl("clientid of control");

ScriptManager.RegisterClientScriptBlock(control, control.GetType(), "whatever", jscript, true);
//we can also use ScriptManager.RegisterStartupScript()