In IIS you can add Server variable to the server variable list in the following steps.
- Go to IIS
- Select the site node for which you want to add variable
- double click URL Rewrite under IIS on the right hand side. If you do not see it you may want to install it from Microsoft site.
- On the right column, select “View Server Variables”.
- If you have any variables you will see it, if you don’t and want to add one, select Add button in the top right column
- Type name name of the variable in the popup dialog and you are all set.