How I can set default reviewers in GitLab Premium?
In Settings → General I have only Merge request (MR) approvals, not reviewers.
There is no such setting in the UI. If you think it would be useful to others, please file a feature request.
As a workaround, you could use the Merge requests API to set reviewers when creating the MR or by updating it after creation.
How you want to achieve the desired result depends on your workflow.
For example, you could have something subscribe to the project webhook which triggers a script that does the API call whenever it sees an open MR event.
Another option, if you run pipelines on MRs, you can have a job run a script which could set reviewers if none are already set via the API.
Yet another option would be to have a specified reviewer set when a merge request is created through the /assign_reviewer @user quick action by adding it to the default and all MR template files.