The Janeway preprints system should immediately create a DOI for your preprint once it has been accepted and published on the preprint server. Sometimes technical issues cause the creation of the DOI to be delayed temporarily.
Each preprint server has an automated process which runs every 30 minutes, scanning published preprints and creating DOIs for any preprint which lacks one. If it has been more than one business day since your preprint was published, and it still does not have a DOI, please contact the preprint server's moderation team for investigation and assistance.