For the legacy IPS type, the serial number of the ASA needs to be attached to IPS service within Cisco's system. You then buy Smartnet for the unit which includes IPS subscription entitlement.
Your reseller should be able to do this.
Be advised that these are end of sales (though not end of new contract attachment yet) and you may get some push back to get a new contract activated for the legacy IPS types. Reference.
The better option would be to go with the FirePOWER module type and FireSIGHT Management Center. It's a superior IPS with other features available as well (URL filtering and Malware protection).