How to screenshare with Agora.io and unity3d

ScreenShare.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using agora_gaming_rtc;
using UnityEngine.UI;
using System.Globalization;
using System.Runtime.InteropServices;
using System;
public class ShareScreen : MonoBehaviour
{
    Texture2D mTexture;
    Rect mRect;
    [SerializeField]
    private string appId = "Your_AppID";
    [SerializeField]
    private string channelName = "agora";
    public IRtcEngine mRtcEngine;
    int i = 100;
    void Start()
    {
        Debug.Log("ScreenShare Activated");
        mRtcEngine = IRtcEngine.getEngine(appId);
        // enable log
        mRtcEngine.SetLogFilter(LOG_FILTER.DEBUG | LOG_FILTER.INFO | LOG_FILTER.WARNING | LOG_FILTER.ERROR | LOG_FILTER.CRITICAL);
        // set callbacks (optional)
        mRtcEngine.SetParameters("{\"rtc.log_filter\": 65535}");
        //Configure the external video source
        mRtcEngine.SetExternalVideoSource(true, false);
        // Start video mode
        mRtcEngine.EnableVideo();
        // allow camera output callback
        mRtcEngine.EnableVideoObserver();
        // join channel
        mRtcEngine.JoinChannel(channelName, null, 0);
        //Create a rectangle width and height of the screen
        mRect = new Rect(0, 0, Screen.width, Screen.height);
        //Create a texture the size of the rectangle you just created
        mTexture = new Texture2D((int)mRect.width, (int)mRect.height, TextureFormat.BGRA32, false);
    }
    void Update()
    {
        //Start the screenshare Coroutine
        StartCoroutine(shareScreen());
    }
    //Screen Share
    IEnumerator shareScreen()
    {
        yield return new WaitForEndOfFrame();
        //Read the Pixels inside the Rectangle
        mTexture.ReadPixels(mRect, 0, 0);
        //Apply the Pixels read from the rectangle to the texture
        mTexture.Apply();
        // Get the Raw Texture data from the the from the texture and apply it to an array of bytes
        byte[] bytes = mTexture.GetRawTextureData();
        // Make enough space for the bytes array
        int size = Marshal.SizeOf(bytes[0]) * bytes.Length;
        // Check to see if there is an engine instance already created
        IRtcEngine rtc = IRtcEngine.QueryEngine();
        //if the engine is present
        if (rtc != null)
        {
            //Create a new external video frame
            ExternalVideoFrame externalVideoFrame = new ExternalVideoFrame();
            //Set the buffer type of the video frame
            externalVideoFrame.type = ExternalVideoFrame.VIDEO_BUFFER_TYPE.VIDEO_BUFFER_RAW_DATA;
            // Set the video pixel format
            externalVideoFrame.format = ExternalVideoFrame.VIDEO_PIXEL_FORMAT.VIDEO_PIXEL_BGRA;
            //apply raw data you are pulling from the rectangle you created earlier to the video frame
            externalVideoFrame.buffer = bytes;
            //Set the width of the video frame (in pixels)
            externalVideoFrame.stride = (int)mRect.width;
            //Set the height of the video frame
            externalVideoFrame.height = (int)mRect.height;
            //Remove pixels from the sides of the frame
            externalVideoFrame.cropLeft = 10;
            externalVideoFrame.cropTop = 10;
            externalVideoFrame.cropRight = 10;
            externalVideoFrame.cropBottom = 10;
            //Rotate the video frame (0, 90, 180, or 270)
            externalVideoFrame.rotation = 180;
            // increment i with the video timestamp
            externalVideoFrame.timestamp = i++;
            //Push the external video frame with the frame we just created
            int a = rtc.PushVideoFrame(externalVideoFrame);
            Debug.Log(" pushVideoFrame =       " + a);
        }
    }
}

Figured it out myself... If you look carefully (unlike me), you'll find a CustomCaptureVideo Example shipped along with the agora sdk for Unity (located under Agora-RTC-Plugin/API-Example/Examples/Advanced/CustomCaptureVideo). If you open CustomCaptureVideo.cs, you'll see a very similar implementation to the gist above. The major difference in my case turned out to be a difference in how raw bytes are being copied at newer Unity versions:

#if UNITY_2018_1_OR_NEWER
                NativeArray<byte> nativeByteArray = _texture.GetRawTextureData<byte>();
                if (_shareData?.Length != nativeByteArray.Length)
                {
                    _shareData = new byte[nativeByteArray.Length];
                }
                nativeByteArray.CopyTo(_shareData);
#else
                _shareData = _texture.GetRawTextureData();
#endif

The external video source is instantiated similar to how I had tried:

private void SetExternalVideoSource()
{
    var ret = RtcEngine.SetExternalVideoSource(true, false, EXTERNAL_VIDEO_SOURCE_TYPE.VIDEO_FRAME, new SenderOptions());
    this.Log.UpdateLog("SetExternalVideoSource returns:" + ret);
}