Axios post is not working in my express Application

I have the following code to get access token from pinterest api as per Oauth when my callback handler is called in my express application, unfortunately the post method always throws 404 error. I can't find any error in my post payload as shown below

        let url = 'https://api.pinterest.com/v5/oauth/token';

        let accessTokenRequestBody = {
            grant_type: "authorization_code",
            code: code, //Received from pinterest at the top
            redirect_uri: encodeURIComponent(redirectUrl) //declared at the top
        }

        const clientIdAndSecretBase64 = Buffer.from(`${clientId}:${clientSecret}`).toString('base64'); //clientId and clientSecret are both declared at the top

        axios.default.post(url, accessTokenRequestBody, { //axios.post was not working so I switched to axios.default.post
            headers: {
                'Content-Type': 'application/x-www-form-urlencoded',
                'Authorization': `Basic ${clientIdAndSecretBase64}`
            }
        }).then((response) => {
            let responseData = response.data;
            console.log(`Pinterest ResponseData = ${JSON.stringify(responseData, null, 2)}`);
        }).catch((e) => {
            console.log(`Pinterest AccessToken Error=${e}`);
        });

This is the error message I keep getting

Pinterest AccessToken Error=Error: Request failed with status code 400

What am I doing wrong here? Thanks for your input.