getFreeTrialPeriod
ISO-8601 trial duration (e.g. "P7D", "P1W", "P1M").
Return
Trial period string, or null if no offer has a trial phase.
Parameters
productId
Subscription product id.
ISO-8601 trial duration (e.g. "P7D", "P1W", "P1M").
Trial period string, or null if no offer has a trial phase.
Subscription product id.