Netflix with coupons?

So I want to set up Netflix, yes you have a free month, but if I specify with voucher code vouchers then they want me from a purchased coupon enter the code, my question: This is then no free months if you have money for a voucher Netflix and then you have a free month and after the free month we then used the coupon to continue watching?

Normally you log in to Netflix as normal, with the payment methods you enter, for example. Direct debit, but never voucher; and then the free month starts automatically.