You have missed adding the Content-Type header in the headers section. Just set the Content-Type header to application/json and it should work.
See the below image:
Also, you might also need to include a CSRF token in the header in case you get an error {"detail": "CSRF Failed: CSRF token missing or incorrect."} while making a POST request using Postman. In that case, add an X-CSRFToken header also with value as the CSRF token value.