How To Exchange Value Between An Input And And Asp:textbox Controler

how to use something like this : ).

<script>// .. rest of the code ...document.getElementById("<%= txt_place_name.ClientID%>").value = document.getElementById('txt_newpl').value;
</script>

Also, wouldn't be easy if you add your <asp:TextBox ... /> to the info window to be completed, like this. In this way you won't complicate your code with unnecessary exchange of values.

<asp:TextBoxID="txt_place_name"CssClass="form-control ltr"ValidationGroup="add"runat="server"></asp:TextBox><script>var tmp = document.createElement("input");
    tmp.appendChild(document.getElementById("<%= txt_place_name.ClientID %>"));

    google.maps.event.addListener(marker, "click", function (e) {
        var infoWindow = new google.maps.InfoWindow({
            content:
            '<div>' + tmp.innerHTML + '</div>'
        })
    });
</script>

You can do this because <asp:TextBox .../> control will be rendered to an html element <input type="text" ... />. This render (transforming) process is happening on sever side.