User clicks on the map, I want to add a marker symbol and a text Symbol displaying the Lat/Long in a text call out.
Here is an example from ArcGIS Desktop Addin.
Adding an individual callout is only possible in Layout (eg. see here: http://pro.arcgis.com/en/pro-app/help/layouts/add-and-modify-graphics-and-text.htm).
Otherwise, for a map, labeling is applied to a layer as a whole. Different labelling classes can be defined as part of a given layer labeling definition (eg based on an attribute or similar) but I don't think that is what you are after.
Retrieving data ...