What happens next
We record your withdrawal with the date and time it reached us, show you that acknowledgement on the next page and send the same text to the address you give. That is the acknowledgement of receipt § 356a (4) BGB requires.
Withdrawing ends the subscription at once. The seller refunds what you paid within 14 days, to the means of payment you used; a proportionate amount for the days you had access may be deducted, as the withdrawal notice says.
A letter or an email works just as well — the address and the model form are in the withdrawal notice. This function exists so that none of that is necessary. It applies within 14 days of concluding the contract; after that the cancellation button ends a subscription at the end of the period you have paid for.