The "Store hash" is an identifier that is required to connect your store to the MessageMedia integration.
To obtain your store hash, start by logging in to your BigCommerce account, and go to Advanced Settings in the navigation bar, click on API Accounts, and then Create API account:
Where it says "API path," just select the character string that appears after /stores/ - this character string is your store hash
In the example shown below, the store hash would be 15tnouweyn/v3/
Paste this to a notepad document for safekeeping as you will need it when connecting your store to your MessageMedia integration.