Create a project
Create a project on launchpad and push your code to trunk.
bzr init
bzr push lp:$PROJECT
Create an empty merge proposal
Branch the code
bzr branch lp:$PROJECT project
cd project
Perform an empty commit
bzr commit --unchanged
Push the code to a branch
bzr push lp:$BRANCH
Propose the branch into trunk
Add to bileto
Add the merge proposal url to the list of merge requests and then select build