These parameters are always mandatory:
Merchant id which you get from your LaterPay merchant backend, which matches your API key.
URL signature (see Signing URLs).
The current time as a number of seconds since the Unix epoch (1970-01-01T00:00:00Z) formatted as an integer. It should be set to the value when the user made the request.
"Jan 1st 2012, 09:00am". This is used to ensure that your API calls are “fresh”, preventing someone attempting to store and re-use a signed URL later.