Cannot retrieve ASP.Net Textbox value in JavaScript function

I’ve got an ASP.Net TextBox that I want to pass it’s text value to a JavaScript function. I’ve tried several different methods, including the one at this answer, Getting Textbox value in Javascript but my value is always returned as undefined. These textboxes are part of an <asp:UpdatePanel> but I have tried this outside of the panel, and receive the same error.

Populating dropdown from SQL Server with ajax in C# and ASP.NET MVC

It seem that debugger doesnt enter at all at the success function. The dropdown im trying to populate is part of a modal in /Dokument/Dokument route when i press a button i get a modal with some fields. One of the fields is a dropdown which has to get values from Tipi table. In the dropdown I see nothing not even the message