Javascript not Not Working.

I want Focus textbox inside Gridview.So I am using Javascript.No Error.But Not Focus.

Protected Sub btntest1_Click(ByVal sender As Object, ByVal e As EventArgs) Handles btntest1.Click
ScriptManager.RegisterStartupScript(Page, Me.GetType(), "HelpScript", "javascript:onClick('" + txt.Text + "');", True)
End Sub

<script language="javascript" type="text/javascript">
function onClick(val) {
debugger;
document.getElementById('maincontent_TabContainer1_TabPanel5_gvFieldDefinition_txtFld_MapPos_'+ val ).focus();
}
</script>