How do you connect an integration that only needs a username/password or API key?

For a plug-in that only needs a username/password or an API key, you enter those credentials in the relevant configuration screen (e.g. add a payment gateway and paste its key/secret; register a delivery company and add its API base URL and key; configure a marketing send provider with its credentials), and the integration becomes available. Default configuration is provided per platform…

For a plug-in that only needs a username/password or an API key, you enter those credentials in the relevant configuration screen (e.g. add a payment gateway and paste its key/secret; register a delivery company and add its API base URL and key; configure a marketing send provider with its credentials), and the integration becomes available. Default configuration is provided per platform where applicable. So connecting a credential-only integration is genuinely a few-fields task — the integration logic is built, and you're supplying the access details it needs.