Creating a command-line tool from an R package is unfortunately not as straightforward as it is for a Python package. It's not a standard option (e.g. there's no usethis function to set it up).
Off the top of my head, the only package I can think of that provides a command-line tool is littler (GitHub repo). Thus you may be able to learn something from that package. Though it is slightly different than your use case, since it is exporting an executable that wraps R itself, whereas you want an executable that wraps your package's functionality.
The simplest option would be to put your script that uses optparse in the inst/ directory of your package, and then provide documentation that instructs your users to copy this file to a directory on their system's PATH (and to make the file executable).