Is there a solution if the SetAttribute of GetElementById does not work?

https://www.paypal.com/myaccount/transfer/buy

I just type characters in the ID element text box.

webBrowser1.Document.GetElementById("recipient").SetAttribute("value", "test@mail");

With the above code, you can enter it on the surface, but the characters are not reflected in the value.
(Also, the text box disappears when you click it.)

Note: Problem symptom has been GIFed.

1st time, input to textbox, click button after disable attribute 入力 Back to before input

Enter //textbox
  dynamic document = webBrowser1.Document.DomDocument;
  document.getElementById("recipient").value="test@mail";
  // disable attribute disabled
  dynamic document = webBrowser1.Document.DomDocument;
  document.querySelector("input[type=submit]") .disabled=false;

The second time, as you asked, you can confirm that the textbox disappears by typing the above code into the textbox and clicking on the textbox.(I think it's not reflected in the value.)

Third time, if you try the above code contents manually, you can see that it will move to the next screen.

If you know the cause, please let me know.
I look forward to your kind cooperation.
visual studio express 2015 for desktop (64bit) windows 10