Adding or tracking Tax collected at Source (TCS) deduction is currently unavailable in QuickBooks Online (QBO). In the meantime, I highly suggest consulting an accountant for guidance with other ways to enter the TCS in your account.
We'll also take note of this as a product suggestion or feature request. You can also visit our Blog site where you can see recent updates and and newly added features in QBO. Here's the link: https://quickbooks.intuit.com/blog/.