Tutorial "GOOGLE_MAPS_API_KEY=

In the ember.js tutorial section "Services and Utilities" at the part where you need to add

GOOGLE_MAPS_API_KEY=<your key here> ember s

I get the error

'GOOGLE_MAPS_API_KEY' is not recognized as an internal or external command,
operable program or batch file.

I know I have followed everything before correctly. And yes i did replace with a valid google maps api key.

Thanks for your help!