In your initialization file, bind the Transient casual-bibtex-tmenu to your key binding of preference in bibtex-mode-map. The binding M-m is suggested so as to not conflict with C-o that is bound to casual-editkit-main-tmenu. (EditKit Install)
(keymap-set bibtex-mode-map "M-m" #'casual-bibtex-tmenu)
casual-bibtex-tmenu is opinionated in making editing and navigation commands emulate a form-based interface (form-based interface) in a bibtex-mode window. The following keybindings are recommended to support consistent behavior between bibtex-mode-map and casual-bibtex-tmenu.
(add-hook 'bibtex-mode-hook 'hl-line-mode) (keymap-set bibtex-mode-map "<TAB>" #'bibtex-next-field) (keymap-set bibtex-mode-map "<backtab>" #'previous-line) (keymap-set bibtex-mode-map "C-n" #'bibtex-next-field) (keymap-set bibtex-mode-map "M-n" #'bibtex-next-entry) (keymap-set bibtex-mode-map "M-p" #'bibtex-previous-entry) (keymap-set bibtex-mode-map "<prior>" #'bibtex-previous-entry) (keymap-set bibtex-mode-map "<next>" #'bibtex-next-entry) (keymap-set bibtex-mode-map "C-c C-o" #'bibtex-url) (keymap-set bibtex-mode-map "C-c C-c" #'casual-bibtex-fill-and-clean) (keymap-set bibtex-mode-map "<clear>" #'bibtex-empty-field) (keymap-set bibtex-mode-map "M-<clear>" #'bibtex-kill-field) (keymap-set bibtex-mode-map "M-DEL" #'bibtex-kill-field)