3scale Developer Portal robots.txt cannot be changed

Issue

We've added the sitemap.xml file to the developer portal and we want to add the following line to the robots.txt file: https://developer.example.com/sitemap.xml
Unfortunately, we cannot find the robots.txt file in the CMS or admin settings. Is there a way for us to update the file manually?
I can see that the file does exist under https://developer.example.com/robots.txt, but we can't find any way to access it from our account.