As seen here - https://www.elastic.co/guide/en/beats/filebeat/current/setup-repositories.html. Also looks like metricbeat has the same issue.

DEB;

echo "deb https://artifacts.elastic.co/packages/6.x-prerelease/apt stable main" | sudo tee -a /etc/apt/sources.list.d/elastic-6.x-prerelease.list

RPM;

[elastic-6.x-prerelease]

I can see that we dynamically figure this out, but I am not sure how that works so didn't want to make a PR and break things :)