You can login to your TimeTap account from Lightning Experience. Simply click the square dots icon in the top left to access all apps:

Then click on the TimeTap Backoffice box to login to TimeTap:

If you are only assigned to one account, then you will be logged into that account directly. If you are an administrator or user across multiple accounts, you'll see a list of your accounts next. You'll then just need to click on the login button next to the client's TimeTap account that you are wanting to work on:

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