In order to add your Quickbooks Online account to Briefcase you'll need to navigate to the 'Accounting' tab 'Settings' in Briefcase. Select 'Quickbooks Online' in the drop down if it isn't already selected and then press the 'Authorise' button in Briefcase to sign in.
Follow the prompts to Authorise Briefcase access to your account.
Products and Services
Under the 'Rates' tab in Briefcase you'll need to ensure your Rates are using the correct Income & Expense Service accounts as found in the Quickbooks Products and Services area. To access the Quickbooks Products and Services area you'll need to login to Quickbooks, navigate to Settings > Products and Services. You can also enter a default product/service on the Accounting tab in case a Rate within Briefcase is not tied to an income/expense account.
Tax codes in Quickbooks must match the Tax Name and tax Rate in Briefcase. This setting can be found in the Settings tab of the main navigation, then under the Accounting tab. Ensure that the any relevant tax codes are entered into Briefcase matching the same naming and tax rate as Quickbooks. These can be found by navigating to the 'GST' menu option on the left panel, and then viewing the GST Rates & Settings Page.