If your port needs to execute commands when the binary package is installed with pkg_add(1) you can do this via the pkg-install script. This script will automatically be added to the package, and will be run twice by pkg_add(1): the first time as ${SH} pkg-install ${PKGNAME} PRE-INSTALL and the second time as ${SH} pkg-install ${PKGNAME} POST-INSTALL. $2 can be tested to determine which mode the script is being run in. The PKG_PREFIX environmental variable will be set to the package installation directory. See pkg_add(1) for additional information.
Note: This script is not run automatically if you install the port with make install. If you are depending on it being run, you will have to explicitly call it from your port's Makefile, with a line like PKG_PREFIX=${PREFIX} ${SH} ${PKGINSTALL} ${PKGNAME} PRE-INSTALL.