How to integrate custom Payment Gateway?

Hello, how can we integrate our own custom payment gateway into Shopify?
I am using NMI and want to integrate the payment gateway into our online store.
What are the requirements and steps?

Hi Esther,

There is a strict process for building payments apps / becoming a Payments Partner that you can learn about here: Get approved for payments

Thank you, it shows that The Partner receives an invitation to submit an application to become a Payments Partner.
How can the invitation be sent to the partner?
Is there a form to request to become a payments partner and receive the invitation?
I’m trying to find out info for our merchant who wants to know how to custom develop the payment gateway to their Shopify

Thank you,

There is no form to request an invite - it’s currently a closed program that is only available to developers that the team are proactively reaching out to.