How do I make qmake install custom widget into integrated designer plugin directory?

I have built a custom widget. I can manually copy it to the Qt Creator bin directory for the integrated designer, and it works as expected (VS 2015, 32-bit, it works fine). My install step copies the DLL to the stand alone designer plugin directory. I have:

target.path = $$[QT_INSTALL_PLUGINS]/designer
INSTALLS += target

in my .pro file.

Is there a way to specify installation into the integrated designer plugin folder? I know I can specify any path I want in target.path, I guess the question is what QMake/Creator variables would get me to the right directory?

Thanks.

Not sure what you mean by "integrated designer plugin folder". That's what QT_INSTALL_PLUGINS is...