How to create custom memstick image of FreeBSD?

Very simple.
  1. Run the setup and choose a USB stick as destination
  2. Boot into your newly-installed system
  3. Install the required packages
  4. Boot into your "normal" system and make an image of the USB stick using dd(1)

Of course you can also do all that within your normal system.
  1. Use dd(1) to create a blank file of the same size as the USB stick and mount an md(4) disk image on it
  2. Use gpart(8) and newfs(8) to create the proper disk and file system structures
  3. Get the distribution files you want (at least base and kernel) and extract them into the mounted image
  4. Use the proper pkg(8) command-line parameters (e.g. --rootdir) to install the packages into the image.