bibtex-include-OPTkey is a variable defined in `bibtex.el'.
Its value is

  • This variable may be risky if used as a file-local variable.

If non-nil, all newly created entries get an OPTkey field.
If this is a string, use it as the initial field text.
If this is a function, call it to generate the initial field text.

You can customize this variable.