This is how I've setup my BitBucket Pull Requests Builder.
* * * * * and it will poll BitBucket every minute to check for PRs.
Setup a shared credential that you can use amongst all projects.
Repo Owner & Name
You should take the data from your repo URL/SSH like:
You can choose to only build specific branches or leave it blank to build all.
CI Identifier & Name
I set this as the name of our CI account we setup on BitBucket. If you create a new BitBucket account just for your pull requests, grant permission to the repositories that it needs. Then whatever username you set that new account to have enter it as the CI Name and CI Identifier.
Finally, leave the bottom two options ticked.
That's the settings that work for us.