A custom set of competitors is not available yet - but will be added with the Content Editor 2.0 update.
Content Editor gives you the possibility to choose from the results of page 1 of the SERPs. This used to be the top 10 but these days, Google is taking up more and more space with snippets, videos, and ads, it can sometimes be 8 or 9 results.
Unfortunately, it is not possible to add a custom URL for comparison within Content Editor.
In 95% of cases, we feel that you really should be able to find relevant competitors on page 1 and if you’re struggling to find competitors with similar intent, similar authority etc, then perhaps you have the wrong keyword chosen or you are going to struggle to rank for this search terms.
However, we are working on a solution for those who need to be able to manually add competitors so watch this space.