First of all you need to add a user to the API, which can be done at Settings > System > API Access, once the user is added the key will be shown along with the user. Keep in mind that API users differ from staff users.
It's not currently possible to have 2 different billing dates using pro rata. New settings would have to be added and you would have to configure 2 days-of-the-month to bill to (1st and 15th) with the largest one overriding the smaller one based on the bill date.
Packages with pro-rata enabled will prorate the service to a specific date, e.g. 1st or 15th of the month, but not both. The cutoff date only determines whether the client should be billed for a partial month, or a full + partial month, the renew date will be the same.