This website should help
ArcGIS Geostatistical Analyst | Evaluate
However, it is a little misleading. There are three options for getting this extension and none of the three are in your case.
Option one is if you have no ArcGIS Software. And it says this:
Receive a full copy of ArcGIS for Desktop at the Advanced license level and selected ArcGIS extensions at no cost for 60 days. |
The word "selected" makes me wonder...
Option two is if you have ArcGIS Single Use License. These are the instructions for that (which I am hoping work for you):
Option three is for a concurrent license and at that point, you have to contact customer service.
Try the idea under Option Two and also check out the link to see if that helps.