Unique bot deep-linking parameter that can be used to generate this invoice.
$startParameter : string
Default
Product name.
$title : string
Default
Total price in the smallest units of the currency (integer, not float/double).
$totalAmount : integer
Default
For example, for a price of US$ 1.45 pass amount = 145.
See the exp parameter in currencies.json,
it shows the number of digits past the decimal point for each currency (2 for the majority of currencies).