Currently, the address and invocation character/keyword fields are only accessible via the edit bookmark dialog. This makes the feature very difficult to discover. Even for users who have used similar features in the browsers, this is an unintuitive place to look for it.
The minimum ask is that the address and invocation character/keyword fields are exposed in the "Add this page to..." (add bookmark) dialog, perhaps initially hidden under a "More Options" or "Search Options" button that reveals these fields when clicked.
Original card: https://orionfeedback.org/d/1548-duckduckgo-style-custom-bang-searches/5
Sibling card: https://orionfeedback.org/d/1602-search-invocation-keywords-think-ddgs-bang-searches-should-be-managed-long-with-search-engines