Drupal 8 Multisite

Firstly sorry if this has already been answered, i did search.
I am running Drupal 8 and wanting to have a multi-site, however I would also like to have a single signin for all.
ie. test1.g4manic.com , test2.g4manic.com.
Currently When switching between sites it doesn’t maintain my login.

Hope this makes sense.


Drupal version: 

Source: https://www.drupal.org/taxonomy/term/22/feed