Let’s break down what is api custom integration. Custom API integration is when you create a new application programming interface (API) that connects to another company's system to make it easier for your systems to work together. This can be done in-house (when you create the API yourself) or with a third party that specializes in creating APIs. The key here is that you're creating a special connection between the two systems - it's not just using an existing interface like Salesforce does with Facebook or Twitter. In a nutshell, custom api integration services is an integration where the data being integrated is not available in a structured format.