I have got it working with the following steps:

In the Auth strategy in nuxt.config.js

• Set the refresh path to /oauth/token (I also added a query string, ?refresh to distinguish out from the login path)
• Add this to the header 'Content-Type': 'application/x-www-form-urlencoded'

Here's the code:

    refresh: {
     url: '/oauth/token?refresh',
     method: 'post',
     headers: { 'Accept': '*/*','Content-Type': 'application/x-www-form-urlencoded' },
    },

I still had problems sending the body in the x-www-form-urlencoded format, so I then created an Axios interceptor plugin to alter the request. This plugin (~/plugins/axios.js) intercepts request made to /oauth/token?refresh and puts the grant type, the refresh token and the client id in the correct format.

Here's the plugin:

var qs = require('qs');

export default function ({ $axios, redirect }) {
  $axios.onRequest(config => {
    if (config.url == '/oauth/token?refresh') {
      const refreshToken = config.data.refresh_token;
      config.data = qs.stringify({'grant_type': 'refresh_token', 'refresh_token': refreshToken,'client_id':'xxxxx-your-client-id-XXXX'})
    }
  })
}

I have got it working with the following steps:

In the Auth strategy in nuxt.config.js

• Set the refresh path to /oauth/token (I also added a query string, ?refresh to distinguish out from the login path)
• Add this to the header 'Content-Type': 'application/x-www-form-urlencoded'

Here's the code:

    refresh: {
     url: '/oauth/token?refresh',
     method: 'post',
     headers: { 'Accept': '*/*','Content-Type': 'application/x-www-form-urlencoded' },
    },

I still had problems sending the body in the x-www-form-urlencoded format, so I then created an Axios interceptor plugin to alter the request. This plugin (~/plugins/axios.js) intercepts request made to /oauth/token?refresh and puts the grant type, the refresh token and the client id in the correct format.

Here's the plugin:

var qs = require('qs');

export default function ({ $axios, redirect }) {
  $axios.onRequest(config => {
    if (config.url == '/oauth/token?refresh') {
      const params = new URLSearchParams(config.data);
      const refreshToken params.get('refresh_token');
      config.data = qs.stringify({'grant_type': 'refresh_token', 'refresh_token': refreshToken,'client_id':'xxxxx-your-client-id-XXXX'})
    }
  })
}