ttl property
How long (in seconds) the message should be kept in FCM storage if the device is offline.
The maximum time to live supported is 4 weeks, and the default value is 4 weeks if not set. Set it to 0 if want to send the message immediately. In JSON format, the Duration type is encoded as a string rather than an object, where the string ends in the suffix "s" (indicating seconds) and is preceded by the number of seconds, with nanoseconds expressed as fractional seconds. For example, 3 seconds with 0 nanoseconds should be encoded in JSON format as "3s", while 3 seconds and 1 nanosecond should be expressed in JSON format as "3.000000001s". The ttl will be rounded down to the nearest second.
Implementation
core.String? ttl;