From this ​the macports-devel mailing list thread:

1. Log in: ​http://build.macports.org
2. Click builders: ​http://build.macports.org/builders
3. Select the builders you want to build on (probably all of the buildports-* builders)
4. Fill out:
   • property1 Name = portlist
   • property1 Value = /port name or list of ports/
5. Click 'Force Build'

You can also click the 'Rebuild' on an individual build page.