Your content-type is incorrect.

You'll need to make it application/json instead of application.json

You can read a bit more about Content-Types in the MDN docs as well as MIME formats for headers or anything else that accepts things in this format. Basically you'll rarely use a period. If ever.

If you're still getting an error after that when trying to insert your data you'll need to expand your question to include what information about how the endpoint you're trying to submit data to expect that input to be handled.