Hello there, roslyn.

Here are the steps to set new GST in purchases in QuickBooks Online.

1. Go to the GST menu.
2. Click the Add tax button in the right corner.
   
3. Select type of tax you want to add. For example, Tax rate.
4. Enter tax name and tick the Purchases box.
5. Enter purchase rate. Then, make sure to select necessary information.
6. Hit Save.
   

To apply this to your purchases, you can follow these steps:

1. Go to the Sales menu and select Product & services.
2. Click New. Then, choose product information.
3. Enter all information.  
4. Scroll down in the Purchasing information section, select the Purchase tax drop-down arrow.
   
5. Select the tax rate you created.
6. Click Save and close.

Please let me know if you have more concerns about adding tax rate in QuickBooks. I'm here to support you an