If you do not want to allow special characters then please refer this link

Allow only AlphaNumeric ( Alphabets and Numbers) characters and space only in TextBox using Javascript

I have used the above code JavaScript in this below article.

Get Value (Text) of dynamically created TextBox in ASP.Net using C# and VB.Net

Please refer this code

C#

private void CreateTextBox(string id)
{
    TextBox txt = new TextBox();
    txt.ID = id;
    txt.Attributes["onkeypress"] = "return IsAlphaNumeric(event);";
    txt.Attributes["ondrop"] = "return false;";
    txt.Attributes["onpaste"] = "return false;";
    pnlTextBoxes.Controls.Add(txt);

    Literal lt = new Literal();
    lt.Text = "<br />";
    pnlTextBoxes.Controls.Add(lt);
}

Add this JavaScript in the page

JavaScript

<script type="text/javascript">
    var specialKeys = new Array();
    specialKeys.push(8); //Backspace
    specialKeys.push(9); //Tab
    specialKeys.push(46); //Delete
    specialKeys.push(36); //Home
    specialKeys.push(35); //End
    specialKeys.push(37); //Left
    specialKeys.push(39); //Right
    function IsAlphaNumeric(e) {            
        var keyCode = e.keyCode == 0 ? e.charCode : e.keyCode;
        var ret = ((keyCode >= 48 && keyCode <= 57) || (keyCode >= 65 && keyCode <= 90) || (keyCode >= 97 && keyCode <= 122) || (specialKeys.indexOf(e.keyCode) != -1 && e.charCode != e.keyCode));
        document.getElementById("error").style.display = ret ? "none" : "inline";
        return ret;
    }
</script>