You need a second product or service that can't be given away for free. It needs to fit perfectly: Buying B with A should make sense in some sub set of cases. B

You need a second product or service that can't be given away for free. It needs to fit perfectly: Buying B with A should make sense in some sub set of cases. B must not be required to use A. I've seen only one free software that solved the puzzle completely perfectly* It does mean it can be done but it seems hard enough that one should probably explore picking B before A. * IIRC It was a desktop Payroll software that only displays ONE advertisement for a lone but only when you have insufficient funds to pay your employees, it does a credit rating based on previous salaries paid and previous financing, there are no forms to fill out as your info is already in the application. I think one can also pay the installments from the application. In that situation the offer seems completely perfect. A button to press that completely solves your problem when you have it.