Is a Sandbox/Testing API and access to it, available?

Hello there,

this has been asked before, but the other topics are all a couple month old.

Do you have a Sandbox/Testing API and if so, can we please get access to it?
What would you need from us to give us access?

Some info of what we want to do:

We are currently refreshing a website for a client, who is using Personio for job offers and we'd like to push new applications through your API rather than using iframes.

That seems easy enough, but we'd like to test/sandbox the implementation before pushing anything into your Live API.

Thanks in advance and
kind regards
Stefan Reichelt