You have the possibility to give your customers a voucher. You decide how many months he can use the service for free. There is the possibility of one, two or three months for both the basic and the premium version. You can create the corresponding code yourself in your extranet. This voucher is generally valid, so you can give it to several customers with the same name.
I recommend to give it only for one month, so you can earn money earlier and the customer has enough time to get to know the system.