In the render location field you have two dropdown menus. One is a triangle that points down and you have the option of “Set Source Folder”. I think this is what you want but it doesn’t seem that WaveLab remembers this setting globally so I would argue that a setting that makes this setting consistent globally would be good for your case. For now, you’d have to make this selection before each render which would be annoying if you are rendering a lot of files.
The other dropdown menu looks like a folder icon and this is a list of recently used paths and in some cases, this list can be useful to quickly point the render location to a folder you recently used. Or, as I often do is point to a recently used render folder but then manually tweak a few characters as needed to change from V1 to V2 or from 1644 or 2496 when rendering different master formats etc.
I just tried to see if there was a special trick to get the “Set Source Folder” option to remain active but as far as I see, you can’t do this. Possibly a good feature request.
Maybe somebody else has a better trick but I certainly don’t see one.