Keypress event for the aspx textbox

You can use the onkeypress Event in JavaScript.
Here is a sample

<asp:textbox id="Textbox1" runat="server" ></asp:textbox>

Javascript function to be called on KeyPress event

function Samplefunction()
    {
        alert('You have pressed a key inside the textbox');
    }

In your code behind file you can call the javaScript function using the below line.

Textbox1.Attributes.Add("onkeypress", "Samplefunction()");

When ever you enter some text in the textbox an alert message pops up.
Refer this link for more information on KeyPress event
onkeypress Event[^]

Hope this helps.