If you need to provide your GST number, you can enter it in the VAT/Tax ID field. Please note, this won't affect the tax calculation.
You can add your GST number by going to Settings -> Billing -> Billing details.
Once done, your VAT/Tax ID number will show up in your invoices.
If you have additional questions about DeskTime billing and GST, feel free to contact our support team at support@desktime.com.