Your Merchant ID (MID) is the unique identifier for your store. It starts with a K or M followed by a series of numbers.
Your MID is shown in the Stores dropdown, available on most pages of the Kustom Portal (for example the Home, Orders, Settlements, and Disputes pages, and under your account in the top-right corner). Open the dropdown to see the MID for each store connected to your login. If you manage more than one store, each one is listed separately so you can confirm which you're working in.