How Do I Populate A Textbox With The Following Variable

Ok - in the code-behind you need to override the onInit method:

protected override void OnInit(EventArgs e)
{
    base.OnInit(e);
    ddlxmlScriptList.SelectedValueChanged += XmlScriptListSelectedValueChanged;
}

Then, when the dropdown value is changed, the method 'XmlScriptListSelectedValueChanged(object sender, EventArgs e)' will run.

I know I am probably talking a bit below your level. Forgive me but I like fully complete answers and you did ask a very generic question :)

I know you know this because this is probably when your code to fetch the xml lives.

If that is not the case then perhaps you have a button. The same principle applies. As long as you have the page still in a loading state (such as when an event is running in the code behind, i.e. XmlScriptListSelectedValueChanged(sender,e)) then this is where and when you need to populate the text-box.

Populating the text-box is simply a case of assigning a string to the textbox's Text property:

XmlScriptListSelectedValueChanged(object sender, EventArgs e){
    
    string theValueINeedInTheTextbox = SomeXmlLoadingStuff(ddlxmlScriptList.SelectedValue);
    tbxTheTextbox.Text = theValueINeedInTheTextbox; 
}

Please let me know if I have answered your question ^_^