How to get it?

You may build it. Or if you don't want to build the app yourself then just pull the right FAP file from latest release and put it to your Flipper Zero device by copying it to /ext/apps/Misc. After that you should be able to run it on your Flipper. Also you may use FlipC.org to get app to your flipper. Just in case if you would like to get app with custom font - consider supporting author here, here, here or in any other way.

To pick the right FAP file you would need to know what firmware your Flipper Zero device is running:

FAP file name	Features	Base firmware	Compatibility
totp_official-stable_fw.fap	All	Official Stable	Official Stable and any other firmware compatible with it
totp_official-stable_fw_badbt-wo-icon.fap	All except of "Automation methods" icon on the token screen	Official Stable	Official Stable and any other firmware compatible with it
totp_official-stable_fw_no-badbt.fap	All except of Bluetooth token input automation (BadBT)	Official Stable	Official Stable and any other firmware compatible with it
totp_official-dev_fw.fap	All	Official Dev	Official Dev and any other firmware compatible with it
totp_official-dev_fw_badbt-wo-icon.fap	All except of "Automation methods" icon on the token screen	Official Dev	Official Dev and any other firmware compatible with it
totp_official-dev_fw_no-badbt.fap	All except of Bluetooth token input automation (BadBT)	Official Dev	Official Dev and any other firmware compatible with it
totp_xtreme_unleashed_fw.fap	All	Xtreme	Xtreme and any other firmware compatible with it (example: Unleashed)
totp_xtreme_unleashed_fw_badbt-wo-icon.fap	All except of "Automation methods" icon on the token screen	Xtreme	Xtreme and any other firmware compatible with it (example: Unleashed)
totp_xtreme_unleashed_fw_no-badbt.fap	All except of Bluetooth token input automation (BadBT)	Xtreme	Xtreme and any other firmware compatible with it (example: Unleashed)

If you are not so sure which firmware your Flipper is running try totp_official-stable_fw.fap first and then if it doesn't work for your then try totp_official-dev_fw.fap.

If you would like to make sure that Flipper Authenticator is completely unable to type code via Bluetooth (BadBT) use the FAP file suffixed with no-badbt. These FAP files are build by completely disabling all the code which deals with Bluetooth, so that you can be sure that Flipper Authenticator is physically can not send anything anywhere via Bluetooth.

If you are fine having Bluetooth token input automation but you just don't like seen that small Bluetooth icon on the token screen use FAP file suffixed with badbt-wo-icon.