Stretch database feature is default disabled. To use it , it must be enabled. In case you try to use it without enabling it , GUI will show a message you to enable the feature
Pre-Check
Change the configuration value of [remote data archive] to 1
use master go sp_configure 'remote data archive',1 reconfigure with override go
Once the configuration value is overridden , the GUI will provide you the option to login to Microsoft Azure
On Successful login.You will see all you subscription as below