Untitled

    <script>
        window.onload = function () {
            var idTextBox = document.getElementById('<%= ID.ClientID %>');
            var submitButton = document.getElementById('<%= Submit.ClientID %>');
            var errorMessage = document.createElement('span');

            // CSS for the error message
            errorMessage.style.color = 'red';
            errorMessage.style.display = 'none';
            errorMessage.style.padding = '5px';
            errorMessage.style.border = '1px solid red';
            errorMessage.style.borderRadius = '4px';
            errorMessage.style.marginLeft = '5px';
            errorMessage.style.marginTop = '5px';

            idTextBox.parentNode.insertBefore(errorMessage, idTextBox.nextSibling);

            idTextBox.addEventListener('input', function () {
                var idValue = idTextBox.value;
                var hasLetters = /[a-zA-Z]/.test(idValue);

                if (hasLetters) {
                    errorMessage.textContent = 'ID should not contain letters.';
                    errorMessage.style.display = 'inline-block';
                    submitButton.disabled = true;
                } else {
                    errorMessage.style.display = 'none';
                    submitButton.disabled = false;
                }
            });
        };
    </script>