With ArcMap or ArcGIS Pro users can add a table with x- and y-coordinates to a map using the 'Display XY Data' (right mouse on table) or the 'Make the XY Event Layer' tool (see: https://www.screencast.com/t/h8hTr4s80K4S). This can also be done with web applications like the Geocortex Essentials HTML5 viewer (see https://www.screencast.com/t/mzNelSnehHxR).
However, this cannot be done at the moment using ArcGIS Online (or Portal for ArcGIS) web maps and web apps. The only way to add points based on coordinates is batch geocoding using the Esri World Geocoder but this only accepts latitude and longitude as coordinates and no XY coordinates.
It would be great if a tool to display XY coordinates could be added, e.g. as a widget to Web AppBuilder.