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.

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

FAP file name	Base firmware	Features	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 "Bluetooth is advertising now" 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_unleashed_fw.fap	All	Official Dev	Official Dev and any other firmware compatible with it, such as Unleashed for example
totp_official-dev_unleashed_fw_badbt-wo-icon.fap	All except of "Bluetooth is advertising now" icon on the token screen	Official Dev	Official Dev and any other firmware compatible with it, such as Unleashed for example
totp_official-dev_unleashed_fw_no-badbt.fap	All except of Bluetooth token input automation (BadBT)	Official Dev	Official Dev and any other firmware compatible with it, such as Unleashed for example

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