Magento – How to add extension attributes to order interface in magento 2.2

Here how Braintree integration looks like (https://developers.braintreepayments.com/start/overview)

So you need do the following

1) Get client token from Magento server. I didn't find rest endpoint for it so I've created a custom extension for this purpose (https://github.com/troublediehard/mma-customapi)

2) Then use sdk and token to get nonce from you card

3) And send your request with nonce data like this

{
  paymentMethod: {
    method: 'braintree',
    additional_data: { 
      payment_method_nonce: nonce, 
    }
  }
}

Right now you are able to access only that api which can be accessed anonymously. Now you are accessing a api which need a authentication key/access token to assure that you are a registered customer who is accessing such api's. So for generating you have to generate a access token using below api :

{base_url}/rest/V1/integration/customer/token

this is a post type api, so you have to pass the username and password in the body,

{
"username":"{your_username}",
"password":"{password}"
}

and hit it, you will get a access token like this :

"6fomgh65em5jaivxqllorc50h2gqrhhe"

Now you have to use this access token in the api you are currently trying to access : In Postman -> go to header and in key = Authorization and in value = Bearer {your generated token} and hit SEND

DONE, That's All.