Thank you for your answer.
You stated, that the plugin intercepts such applications before they are saved to WordPress. The problem is, that after passing an application to the API, the plugin removes the application from WordPress. After that, when a new application has been submitted, the plugin has no applications to check against if they already exist in Personio and will add a new application to WordPress. When trying to transfer this new application to Personio, the API throws the error stating the application already exists in Personio.
Is there a way to intercept even further?