When you use the right-click context menu to “Copy custom field name”, you will get the input field id value, which as you noted, has the form usaa-form-v5-10-1-input-y7598elcdbc and will not be helpful. However, if you examine the HTML source code (e.g., using Inspect in the right-click context menu in Chrome), you will find that the input field also has a name parameter that can be used for custom field matching. In the case of the “Online ID” field, the name is memberId, and in the case of the password field, the name is simply password.
I have verified that in a Chrome browser, it works with linked custom fields named memberId and password linked to the Username and Password, respectively. You may have to manually trigger a second auto-fill (using the Ctrl+Shift+L keyboard shortcut) to fill the password field after clicking “Next”.