interface Api {

    @FormUrlEncoded
    @POST("api/v1/access_token")
    fun retrieveToken(@Field("grant_type") authCode: String,
                      @Field("code") code: String,
                      @Field("redirect_uri") redirectUri: String = REDIRECT_URI): Call<ResponseBody>

}

class RetrofitStore {

    private val client = OkHttpClient.Builder()
            .addInterceptor(HttpLoggingInterceptor().apply {
                level = HttpLoggingInterceptor.Level.BODY
            })
            .addInterceptor(TokenHeaderInterceptor())
            .build()
    val retrofit: Api = Retrofit.Builder()
            .baseUrl(BASE_URL)
            .client(client)
            .addConverterFactory(GsonConverterFactory.create())
            .build()
            .create(Api::class.java)
}

class TokenHeaderInterceptor : Interceptor {
    private val settings = UserSettings()
    override fun intercept(chain: Interceptor.Chain): Response {
        val request = chain.request().newBuilder()
        request.addHeader("Authorization:", "Digest user=\"$APP_ID\" password=\"${settings.getUserCode()}\"")
        return chain.proceed(request.build())
    }

}


вот так дёргаю:

private fun retrieveAccessToken(code: String) {
        val retrofit = RetrofitStore().retrofit
        retrofit.retrieveToken(AUTH_CODE, code).enqueue(object : Callback<ResponseBody> {
            override fun onFailure(call: Call<ResponseBody>, t: Throwable) {

            }

            override fun onResponse(call: Call<ResponseBody>, response: Response<ResponseBody>) {
            }

        })
    }