Image insertion problem with TinyMCE

I try to insert an image from TinyMCE editor.

When I click on the “insert image” logo, there is still no browse button (this is a shame in 2020 !..) but there is a field named “source” in which i suppose I have to fill the exact URL of the image.

So for example: “/localhost/grav/images/myimage.jpg” but it does not work at all.

What should be written in this field ?