Here are two links:

https://help.medium.com/hc/en-us/articles/225168768-Writing-and-publishing-your-first-story

https://help.medium.com/hc/en-us/articles/213904978-Add-a-draft-or-post-to-publication

You have to write a draft and leave it unpublished. Then you click on the three dots and click Add To Publication and then choose the appropriate pub. After you do that, you click Submit. It sounds more complicated than it is but it can be intimidating at first. Make sure to add tags and include credits for any artwork you include.