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To set the value of a textbox sequentially on the server-side, followed by the client-side, and then again on the server-side, you can use a combination of server-side code and JavaScript.

1. On the server-side, set the initial value of the textbox:

TextBox1.Text = "Initial value";

1. On the page's HTML, add an event handler to the textbox to update its value on the client-side:

<asp:TextBox ID="TextBox1" runat="server" onchange="updateValue(this)"></asp:TextBox>

1. On the client-side, create a JavaScript function that updates the value of the textbox:

function updateValue(textbox) {
  // Update the value of the textbox
  textbox.value = "Client-side value";

  // Call the server-side method to update the value again
  PageMethods.UpdateValue(textbox.value, onSuccess);
}

function onSuccess(result) {
  // Update the value of the textbox again on the server-side
  document.getElementById('<%=TextBox1.ClientID%>').value = "Server-side value";
}

1. On the server-side, create a method that updates the value of the textbox:

[WebMethod]
public static void UpdateValue(string value) {
  // Update the value of the textbox
  TextBox1.Text = value;
}

Note: To use PageMethods in your client-side JavaScript, you need to add the following script manager to your page's HTML:

<asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true"></asp:ScriptManager>