You might contact the publication and ask them how the file should be set up.
An easy trick for doing spot colors in Photoshop is to create the file in CMYK, and only use the Cyan and Black plates or channels, making sure the Magenta and Yellow channels are empty. Then tell them “use PMS 300 (or whatever) instead of Cyan.” But since the file is being dropped into a publication, this shortcut may not work for them.
You may need to do it the more “proper” way – create a multichannel file and save it as a Photoshop DCS 2.0 EPS. Basically, all your black/greyscale printing will be on a black plate or channel, and everything blue will be on a spot color channel. Here’s a link to an article that explains the process:
https://www.creativepro.com/story/feature/19805-2.html