<script type="text/javascript" language="javascript">
    function Func() {
        alert("hello!");
    }
</script>

public override Umbraco.Forms.Core.Enums.WorkflowExecutionStatus Execute(Umbraco.Forms.Core.Record record, Umbraco.Forms.Core.RecordEventArgs e)
    {
        var page = HttpContext.Current.CurrentHandler as System.Web.UI.Page;

        page.ClientScript.RegisterStartupScript(GetType(), "myFunction", "Func();", true);
        return WorkflowExecutionStatus.Completed;
    }