Let me assist you from here, Thanhphuongy.

QuickBooks Online integrates with hundreds of apps. You'll want to reach out to Clockify support since it's a third-party app. They'll help you with any integration issues.

Concerning time tracking, it's free if you'll use the basic time tracking in QuickBooks Online. However, if you opt for QuickBooks Time with more tracking features, it's a subscription-based app. Either way, both of them let your contractor access timesheets from QuickBooks Online. If you want to use the basic time tracking, you'll be adding the contractor as a user. If you wish to subscribe to the latter, feel free to check this reference to help you manage the time activities: Track and manage QuickBooks Time in QuickBooks Online and QuickBooks Online Payroll.

Feel free to get back to this thread if you have additional questions. We'll reply as soon as we can. Take care!