Question: Title mostly says it. How can you trigger a Jenkins multibranch pipeline project build from a remote git repository?

The "Trigger builds remotely" build trigger option does not seem to work, since no tokens that you set are saved.

Answer: 
At the moment (Jenkins 2.22) the "Trigger builds remotely" build trigger option is visible in the multibranch pipeline job configuration, but does not work (if you check it and specify a token, it gets reset after saving anyway). According to [this](https://issues.jenkins-ci.org/browse/JENKINS-35310), it is intentional that the trigger cannot be set, but a bug that it appears as an option at all. 

In the same thread they explain how to trigger builds for each individual project (branch) in a multibranch pipeline project. What I needed was a dynamic setup that would work for branches created after setting up the trigger, so rather than the suggested endpoint from the thread (`/job/project-name/job-name/build`, which should have been `/job/job-name/project-name/build` , since projects are created from branches in a job), I found that the endpoint to use is `/job/job-name/build`. In order for that to work you have to create a user with an API token (go to Manage Jenkins -> Manage Users -> Gear icon -> Show API Token), and use those as username and password in your request.

The solution might be obvious for those used to working with Jenkins REST API, but when you are new to both multibranch pipeline projects and the REST API, it doesn't hurt to be explicit.
I just recently got over this hurdle and would like to share my note. 

In my configuration (Jenkins 2.60.2), there is no way for me to enable the Trigger builds remotely (e.g., from scripts) option since I can only "View Configuration". This prevented me from trigger the pipeline by issue HTTP GET to end point (/job/project-name/job/job-name/build).

However, I found out that I can always trigger by issue HTTP POST to the end point.

You will have to provide a valid crumb in the request or disable the "Prevent Cross Site Request Forgery exploits" option in Configure Global Security.

Regards,