Help with generating access tokens in Node.js

I recommend using the official Dropbox JavaScript SDK if possible, as it will do much of the work for you. Alternatively, it may also just serve as a good example even if you can't use it.

Otherwise, refer to the documentation for /oauth2/token. It contains examples of calling the endpoint for different scenarios that you can translate to your platform.

For example, the error you posted here is indicating that the call failed because it's missing the required parameter "grant_type". I see you're attempting to set that, but you're trying to do so as JSON; this endpoints takes these parameters as application/x-www-form-urlencoded POST parameters, not JSON.