Where attribute names need to be entered, for instance in the Key Attribute selector, setting up new columns in column view or picking an attribute in the system or User Inspector tabs, Tinderbox will offer autocompletions from existing defined attributes.
If the part-typed string only has one possible auto-match, that match is used to complete the current string. If this completion is not wanted, e.g. due to a typo, use the Escape key (not the Backspace) to roll back the auto-complete after which other keys such as Backspace will work as expected to edit the user-typed part of the string.
When searching for attributes in the Get Info/attributes popover, Tinderbox looks for the search string at any position in the note. Thus, 'URL' offers potential completions of $URL, $ReferenceURL, $SourceURL, and $NoteURL.