Jenkins Multibranch Pipeline with Github source - Discover pull requests by target branch

Question

I'm trying to define my Multibranch pipeline in Jenkins, with a 'Github' source. Within the 'Behaviors', I'm looking for an option to define my strategy to discover pull requests that have a specific target branch (In my case - I want to the branch to be 'master'). Is that possible within the Jenkins Multibranch Pipeline UI?