An integration is a relationship between systems, not just a button that sends data somewhere else. The happy path is usually easy; reliability depends on what happens when a request is slow, duplicated, rejected, or changed.
Clarify the contract
Document authentication, required fields, rate limits, response formats, error codes, webhooks, test credentials, and versioning. Decide which system owns the source of truth and how a failed request will be retried or surfaced to a person.
Protect the people and data
Send only what is needed, restrict secrets, log useful events without exposing sensitive data, and define how access is revoked. Test with realistic failures before launch.
Custom application development can include this reasoning as part of the build rather than leaving it to a rushed final week.