In an invoice if I have a product which is out of scope of tax and if I want to do a markup on that and add 18%gst only on the markup. Then how can i show it on invoice?

Welcome to the Community, @suvidhijalori. I'm here to help you add the markup and show it to your invoice.

You'll have to manually edit the Amount of your product and add the 18% GST. That's the only way to show up the increase of the markup in an invoice.

For example, the amount of your product is 200 then you'll have to add 36 for the markup. Let me show you how;

1.  
2. Go to the +New icon.
3. Click the Invoice button.
4. From the Customer ▼ dropdown, select a customer.
5. From the Product/Service column, select a product or service.
6. In the Amount, column add the markup increase.
7. Select Save and close or Save and send.

See this article for details: Create invoices in QuickBooks Online.

You can also turn on the automatic markup feature for your billable expense. For more information, click this article: Enter billable expenses. 

Once everything is all set, you might want to refer to this link for guidance in receiving payment and how to personalise your invoice: 

• Record invoice payments in QuickBooks Online
• Customise invoices, estimates, and sales receipts in QuickBooks Online

If you need further assistance or inquiries about managing customer transactions in QBO, I'll be right here to help. Take care and stay safe always, @suvidhijalori.