Set HTML TextBox value inside content page using JavaScript

Hi moquamar,

Please call JavaScript function on Button click event.

Refer below sample.

Note: Here I am creating this sample with the help of JavaScript document.getElementById not working with Master Page and Content Page in ASP.Net

HTML

<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
    <input id="MyInputBox" name="MyInputBox" type="text" runat="server" />
    <input type="submit" value="Submit" onclick="callAlert()" />
    <script type="text/javascript">
        function callAlert() {
            alert('Calling js from Content Page.');
            document.getElementById('<%= MyInputBox.ClientID %>').value = "My value 1";
        };
    </script>
</asp:Content>

Screenshot