Unlike all of the services we have seen so far, the Geometry Service is not dependent on some underlying resource. That is to say, it is not something that you must first create in ArcGIS Pro and then publish to ArcGIS Server.
You may say, and you would be correct, that you haven't had to do that for any of the services we have been using in this book. However, the point is that someone, somewhere, has done so. The basemaps you have been consuming in the web applications you have created so far all started off as a map document somewhere, and have been made available to you as the result of someone publishing that document to a publicly available ArcGIS Server instance. The geoprocessing services you have accessed were similarly conceived in Model Builder, or as a Python script.
This is not the case with the Geometry Service. To create a Geometry Service, all you have to do is get your ArcGIS Server administrator to enable it and you are good to go!
Once you have a running...