How to set Environment Variable for your Account

Control Panel V5 > Hosting Control Panel > Advance
The Environment Variable is based on IIS 10, which will be set on your Application Pool level, you are able to use it on any sites under the same application pool.
Please note the variable name must be unique in the list.
Please refer to the below screenshot and go to Control panel>>Advance>>Pool Manager>>Actions>>Environment Variables to set that.
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