TrackRequestBodyInitializer.cs

using Microsoft.ApplicationInsights.Channel;
using Microsoft.ApplicationInsights.DataContracts;
using Microsoft.ApplicationInsights.Extensibility;
using Microsoft.AspNetCore.Http;

namespace YourNamespace
{
	//	https://stackoverflow.com/questions/42686363/view-post-request-body-in-application-insights
	public class TrackRequestBodyInitializer : ITelemetryInitializer
	{
		private readonly IHttpContextAccessor httpContextAccessor;

		public TrackRequestBodyInitializer(IHttpContextAccessor httpContextAccessor)
		{
			this.httpContextAccessor = httpContextAccessor;
		}

		public void Initialize(ITelemetry telemetry)
		{
			if (telemetry is RequestTelemetry requestTelemetry)
			{
				var context = httpContextAccessor.HttpContext;
				var request = context.Request;

				if (request.CanReadBody())
				{
					const string jsonBody = TrackConstants.JsonBody;

					if (!context.Items.ContainsKey(jsonBody))
					{
						return;
					}

					if (requestTelemetry.Properties.ContainsKey(jsonBody))
					{
						return;
					}

					var body = context.Items[jsonBody] as string;
					requestTelemetry.Properties.Add(jsonBody, body);
				}
			}
		}
	}
}