1. Go to Accounting > Chart of Accounts and find the category (for example Purchases - Food & Beverage).
2. Click the lines icon titled Auto-categorization rules on that row. The Auto-Categorization Rules window opens showing Rules for <category>.
3. Under Keyword Rules, type the text to look for in the box (placeholder e.g. AMAZON, WHOLE FOODS, up to 400 characters) and click Add.
4. The keyword appears in the table with a Matched count of 0. Repeat for more keywords, then click Done.

The rule takes effect for transactions imported from now on (when Auto-categorize Transactions is enabled) or when you run Re-categorize All on the Transactions page. Adding a rule does not re-categorize existing transactions by itself.