Unfortunately, Scheduled pipeline is realized through GUI rather than .gitlab-ci.yml file in GitLab
You may find this document useful if you want to use GUI for schedule pipeline. Configuring in GUI is simpler than yaml coding.
If you really want it to be configured in yaml file, you may create an issue in GitLab repo. Actually, there's already an issue on this topic. It should not be a very difficult technically since it is only about keywords and parsing. Whether GitLab will do it or not is another story; so you'd better prepare a good scenario to persuade the GitLab team why you want this to be done.