You will need to provide a personal token for Harvest and the account ID you want to use by default.
You can create a token by visiting: https://id.getharvest.com/developers
You can get the Account ID on the page of the token after you created it.
Installation
brew install ctrlaltdev/tap/harvest
or
brew tap ctrlaltdev/tap
brew install harvest
Usage
Just run harvest.
If you never set it up before, it will ask you for your personal token and the account ID