Account menu, then Integrations. There are three things on that page and it is worth being blunt about what each one does.

What exists today
| Route | What it does | State |
|---|---|---|
| API key | Read your candidates and their results from your own systems | Available |
| Webhooks | We POST to your URL when a candidate registers or completes a test | Available |
| ATS connectors | Named connectors for applicant tracking systems | Not available yet |
The API is read-only
The API has a single endpoint, which fetches candidates and their assessments and attempts. You cannot create an assessment, invite a candidate or unlock a result through it. Those are things you do in the app.
If you have read otherwise elsewhere in the product, this article is the accurate version. More endpoints may be added; none exist today.
Choosing between the options
You want results in your own reporting. Use the API. Poll it on whatever schedule suits you, or use a webhook to know when to poll.
You want to react the moment something happens. Use a webhook. It reaches you within seconds of a candidate registering or submitting a test, without you asking.
You want candidates invited automatically from another system. That is not possible yet through either route. What works today is putting the assessment link into the templated email your other system already sends. See where to use your assessment link.
You just want the data once. Do not build anything. Export a CSV. See download and export results.
A one-off need does not justify an integration
Most teams who ask about the API want a spreadsheet, and an export gives them one in ten seconds. Build against the API when you need results to arrive continuously, in a system other people use, without anyone remembering to download anything.
Access is account-wide
Anyone on your team can see, reveal and regenerate the API key, and anyone can change the webhook endpoint. There is no separate developer role. See what your team can see.
Telling us what you need
If your system is not listed and you would use a connector, say so: support@testcandidates.com. That genuinely feeds the order things get built in.