Environment variables are a great way to store data to not only use to post requests but also to compare against returned values.
The main command is
To re-use a saved variable, use the prefix environment. before the saved variable name.
To use the variable in the url or headers, put the name of the saved variable in braces
and that’s about it.
Very simple, but also very powerful allowing you to change a re-used variable in one place which is the Manage Environments which is accessible through the gear icon to the top right of postman.
For more information, see the postman documents: Postman documents on Environment Variables.