Tested working. Can be used on existing Google accounts. Cancel anytime and still keep the 3 months free subscription.
If you get the message, “ It seems that you have used up all your allocated codes.
Please wait for another hour”, change the 3 numbers on the end of the OP link URL address to something random e.g. 432. The 3 digits need to be less than 821.
Full credit to Chollometro.