This request will return all the details of a specific merchant account. If you need to get multiple merchants' info at once, use the Get Merchant List request.
The `gateway_id` in the POST URL refers to the "gateway ID" for the merchant you are updating, so this will change with each new merchant. This is accessible from the Merchant Details page in the partner portal, as well as returned in the Merchant Boarding API when creating a merchant account.
The `domains` value is expected to be an array of domains the merchant would like to be allowed to use Apple Pay.
> Please be aware that the domains posted to this endpoint should be the full list of domains allowed for this merchant. For example, if you made the same example request again, but only included abc.com as a domain, then example.com and store.acme.com would be removed.