How I do do a fetch request with custom headers to a 3rd party API?

@avinashmurthy you are right that this cURL command translates to what you tried, checked in Convert curl commands to JavaScript .

fetch('https://<dunzo_host>/api/v1/token', {
    headers: {
        'client-id': '<CLIENT_ID>',
        'client-secret': '<CLIENT_SECRET>',
        'Accept-Language': 'en_US',
        'Content-Type': 'application/json'
    }
});

But I would try adding those client properties to the body anyway.