It was actually really easy. I just had to revive my Pocket account on the iPhone and login to it on the Kobo Clara as well. It is all prepared on Kobo and when you then sync the stored articles appear.
To have the Medium articles appear on my Kobo I just had to choose one and share it through Pocket.
On the iPhone
Here is an example of the workflow. I selected this article in the Medium app and then pressed the share button on the bottom:
A share dialogue appears and I find the Pocket icon. In my case it was there right away:
Once tapped there is a confirmation and the iPhone part is done:
On the Kobo Ereader
Next I open the ereader and go to articles. The ereader is connected to WiFi and I tap sync. The icon on the top row that looks like this 🔄
And there the article appeared:
Ready for reading.