If you are intending to add a script block to the rendered page when WebGrid do FlyPostBack, please try to use InvokeScript method of WebGrid in OnInitializePostBack server-side event of WebGrid.

Please follow the step-by-step below carefully in order to add a script block to the rendered page.

1. Add a JScript file into the project.
   This JScript file is the file that contains the script block that about to be added to the rendered page.

MyScript.js

function ShowKeyCode() {
    var textBox = document.getElementById("Message");

    alert(event.keyCode);
    
    return true;
}

2. Use InvokeScript() method in OnInitializePostBack server-side event of WebGrid.
   In the InvokeScript() method, call IS.AddScript and pass the path of the JScript file in the step #1.

Default.aspx.cs

protected void WebGrid1_InitializePostBack(object sender, ISNet.WebUI.WebGrid.PostbackEventArgs e)
{
    WebGrid1.ClientAction.InvokeScript("IS.AddScript('MyScript.js');");
}

If the InvokeScript doesn’t suit with your scenario, please kindly let us know.

Edited reason: add and correct step-by-step information.