If you want to host a Bazaar branch on Launchpad, you can push it straight to Launchpad without first registering.
Simply open up your terminal, change to the directory that contains your Bazaar branch and type:
bzr launchpad-login userid
bzr push lp:~userid/project-name/branch-name
Replace userid with your Launchpad id, project-name with the project's Launchpad id and then chose whichever branch name you want.
Read more about pushing Bazaar branches to Launchpad >
If you register the branch in Launchpad's web interface, before pushing anything to it, you'll need to use a slightly different Bazaar command the first time you push anything to the branch:
bzr push lp:~userid/project-name/branch-name --use-existing-dir