One of the authors here. In GitLab CI, you can create that manual trigger button by including the `when: manual` parameter in your .gitlab-ci.yml. You can also restrict who can click that button for protected pipelines, such as those that deploy to production. (https://docs.gitlab.com/ee/ci/yaml/#whenmanual)

For GitHub Actions, try `workflow_dispatch` (https://github.blog/changelog/2020-07-06-github-actions-manu...).