In the normal jobs (freestyle) or the pipeline jobs (non-multibranch), there is the "Run" button in the Jenkins Blue Ocean Pipeline.

In the multi-branch pipelines, the "Run" button is missing, and you need to go to the branches tab and press the small "|>" button, which is harder to know what it means.

I know that the question would be: to what branch that "Run" button to point at? I suggest having it pointed to the master branch by default and have a pipeline configuration option to change that default behavior. Or reuse the favorite plugin and point to the first favorite.