The search bar on tiktokdl.com
is not working for me, but you can get the userID for a specific user through the app.
Search for them on the app, then share their profile through a browser link, and the userID will be in the redirected URL.
So, for Alex for example:
On the app, search for @thiefof_heart
Then Share Profile -> Copy Link, which will give you a URL like this: http://vm.tiktok.com/JkbQGq/
The link above will redirect here: https://m.tiktok.com/h5/share/usr/60290745210785792.html
Now that you have the userID, you browse all the available videos for that user in your browser at: http://tiktokdl.com/user/60290745210785792.html