Adding environment variables to APP Pool

Source: https://docs.microsoft.com/en-us/iis/configuration/system.applicationhost/applicationpools/add/environmentvariables/

The following example adds an environment variable to an application pool.

<applicationpools>
<add managedpipelinemode="Classic" managedruntimeversion="v4.0" name="Contoso">
<environmentvariables>
<add name="foo" value="bar"></add>
</environmentvariables>
</add>
</applicationpools>

you can also add environment vars using appcmd tool

%systemroot%\system32\inetsrv\appcmd.exe set config -section:system.applicationHost/applicationPools /+"[name='Contoso'].environmentVariables.[name='foo',value='bar']" /commit:apphost