How to correctly reference WRKSRC of another port?

Hi everyone,

I'm currently working on a port (my first ;-)) which has a (optional) 
build time dependency on the sources of math/nauty.

Currently, I've hardwired '/usr/ports/math/nauty/work/nauty27r1' as 
argument to the configure script, but that's clearly not the way to do it.

The Porter's Handbook mentions using WRKDIRPREFIXPORTSDIR [0], but that 
does not work. A search for WRKDIRPREFIXPORTSDIR in /usr/ports/Mk comes 
up empty. Do I explicitly have to define this variable?

I've set NAUTY_BUILD_DEPENDS=nauty>0:math/nauty:extract. How do I 
correctly reference nauty's WRKSRC?

It would be nice if anyone could point me in the right direction.

Thanks in advance!

Philipp


[0] 
https://docs.freebsd.org/en/books/porters-handbook/porting-dads/#porting-wrkdirprefix