(TEMPLATE)
POST /auth/token HTTP/1.1
Host: oauth.wildapricot.org
Authorization: Basic QVBJS0VZOm85c2U4N3Jnb2l5c29lcjk4MDcwOS0=
Content-type: application/x-www-form-urlencoded

granttype=clientcredentials&scope=auto

(My Attempt)--------------------------------------------------------------------

object WAApiCallService {
private const val APIKEY = "xxxxxxxxxxxxxxxxxxxx"
private const val BASEURL = "https://oauth.wildapricot.org"
private val AUTH = "Basic" + Base64.encodeToString(APIKEY.toByteArray(), Base64.NOWRAP)
private const val CONTENT_TYPE = "application/x-www-form-urlencoded"

private var api:WAApiCall? = null

private fun getWAApi(context: Context) : WAApiCall {

    if(api==null){
        val OkHttpClient = OkHttpClient.Builder()
        val logging = HttpLoggingInterceptor()
        logging.level = HttpLoggingInterceptor.Level.BODY
        if(BuildConfig.DEBUG){
            OkHttpClient.addInterceptor(logging)
        }
                //define and add a cache
        val cacheSize = 5 * 1024 * 1024L // = 5mb cache
        val cache = Cache(context.cacheDir, cacheSize) 
                //add cache to client
        OkHttpClient.cache(cache)

            //add custom interceptor
        OkHttpClient.addInterceptor{chain ->
            //get the outgoing request
            val request = chain.request()

           val newRequest = request.newBuilder()
                .addHeader("Authorization", AUTH)
                .addHeader("Content-type", CONTENT_TYPE)
                .build()
            chain.proceed(newRequest)

        }
        api = Retrofit.Builder()
            .baseUrl(BASE_URL)
            .addConverterFactory(GsonConverterFactory.create())
            .client(OkHttpClient.build())
            .build()
            .create(WAApiCall::class.java)
    }
    return api!!
}

fun call(context: Context) =
    getWAApi(context)

}

(The Call Interface)-------------------------------------------------------
interface WAApiCall {

@POST("auth/token HTTP/1.1")
fun callPost(@Body body:String ): Call<AuthToken>

}

(RESPONSE DATA CLASS)--------------------------------------------------------
data class AuthToken(
@SerializedName("accesstoken")
var accessToken: String,
@SerializedName("expiresin")
var expiresIn: Int,
@SerializedName("permissions")
var permissions: List<Permission>,
@SerializedName("refreshtoken")
var refreshToken: String,
@SerializedName("tokentype")
var tokenType: String
)

(FUNCTION IN MAIN ACTIVITY THAT I CALL)-----------------------------------------

fun testRequest(){
val call = WAApiCallService.call(this)
call.callPost("granttype=clientcredentials&scope=auto&obtainrefreshtoken=true")
.enqueue(object: Callback<AuthToken>{
override fun onFailure(call: Call<AuthToken>, t: Throwable) {
Log.i("FAILURE", t.localizedMessage)
}

        override fun onResponse(call: Call<AuthToken>, response: Response<AuthToken>) {
            Log.i("SUCCESS", "TOKEN = ${response.body()}")
            Log.i("SUCCESS", "${response}")
        }
    })

}

(THE RESPONSE I GET)-----------------------------------------------------------
I/SUCCESS: TOKEN = null

I/SUCCESS: Response{protocol=http/1.1, code=404, message=Not Found, 
  url=https://oauth.wildapricot.org/auth/token%20HTTP/1.1}