You will need a Mongo database configured for your Rocket.Chat application, for better performance as meteor hosting uses AWS it is advice you setup your database in the same AWS region.
Create a settings.json file to set environmental variable as follows:
region is ‘galaxy.meteor.com’ for US East, ‘eu-west-1.galaxy.meteor.com’ for EU West, and ‘ap-southeast-2.galaxy.meteor.com’ for Asia-Pacific.
hostname is the fully qualified domain name where you’re planning to host your application (using in the .meteorapp.com hostname format, in this example).
filepath.json is the path to your settings file (for example, ‘./settings.json’).
For more information on these steps check out galaxy's doc​