The current github-plugin does not support configuration of the projectURLStr for a multi-branch pipeline. Standard Declarative pipelines allow configuration through the normal build options in the user interface but multibranch pipelines may only be configured through the pipeline script (in a Jenkinsfile).

The current github-plugin does not expose the GithubProjectProperty JobPropertyDescriptor through a Symbol, and the old method of specifying a properties([$class: 'GithubProjectProperty']) as recommended by the snippet generator is not supported.

See StackOverflow post: https://stackoverflow.com/questions/61761031/specifying-a-github-project-url-in-a-jenkins-multibranch-pipeline for a more thorough description of the problem.