I would love to add another "build" on my own from within a pipeline.
On a pull request, the UI shows on the right top hand a summary of the builds (e.g. "3 of 3 passed") 
When clicking, there is more detail:

Builds number one and two are created by Bitbucket Cloud Pipelines Plugin itself for tow piplines: Pull Request and Branch.
Build number three is from a remote service, here SonarCloud.
Similar to what SonarCloud does here, I would like to learn which APIs there are to create a build entry here from within my pipelines.
I've looked into the REST API but was a bit lost as I couldn't find anything useful. Maybe this is for Webhooks? But then I didn't know where to find Webhooks (and I guess these are the other way round).
Any pointers would be great. I can imagine it's as easy as just firing some curl calls.