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.

Thanks.

Drupal version: 


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