customize-push-and-save is an autoloaded compiled Lisp function in `cus-edit.el'.

(customize-push-and-save LIST-VAR ELTS)

Add ELTS to LIST-VAR and save for future sessions, safely.
ELTS should be a list. This function adds each entry to the
value of LIST-VAR using `add-to-list'.

If Emacs is initialized, call `customize-save-variable' to save
the resulting list value now. Otherwise, add an entry to
`after-init-hook' to save it after initialization.