Creating hosted payment provider

[puresystems]

Please summarize your question in one sentence

We have a client who wants to integrate the cart with a hosting payment page (similar to PayPal, etc) where the customer would be sent to another site to make payment from the HC checkout page, and then returned to HC to complete the purchase.

Give a more extended description

We have downloaded and implemented the Custom Payment Method example from here:
https://hotcakescommerce.zendesk.com/hc/en-us/articles/204725899-Custom-Payment-Method-Example

But we're unsure how/where to get the checkout page to POST the data to the 3rd party payment page (as the PayPal Express provider does).

Any advice would be gratefully received.