If you are an existing customer, you can refer a new customer and receive one month free when they create a subscription. Your referral code is your chosen subdomain. 

If you are a monthly subscriber when you refer a new customer, you can pause the subscription whenever you want to and wait a month before resubscribing. 

If you are a yearly subscriber this might be more complicated. Issuing such a small refund ($2.16) would be too costly. Instead, you can wait for the yearly subscription to end, then simply wait a month before resubscribing.