If you want to use ExpressVPN with OpenBSD, you'll have to select one of the available client technologies that will interoperate with that company's servers.
The lastpass-cli utility is described as "an open-source command-line interface to the popular LastPass password management service." The most recent release of that tool is 1.3.3, which was released on April 15, 2019. It is copyright 2014-2019, by LastPass. (Source
https://github.com/lastpass/lastpass-cli)
Firefox is available from packages, either as "firefox" or "firefox-esr" (for the extended support release) and with a wide variety of available language packs, also. Please read the OpenBSD "package README" file at /usr/local/share/doc/pkg-readmes/firefox, as the application has been additionally secured with OpenBSD-specific pledge(2) and unveil(2) services to limit its behavior and access. These changes have operational impact -- such as directory access -- that you will need to be aware of.