You must first add the field to the form in order to include the client's GST number:
Go to the "Setup".
Click on the "Custom Fields".
Enable the GST number, as shown below:
4. Go to the "Clients".
5. You must select the client, whom you need to add the GST number.
6. Click on the three dots(...).
7. Click on the "Edit".
8. Add the "GST number", Click on the "Save".
Adding a new client also allows you to include the GST number.