Hi .. How do I add Company's GST Registration number in Tax invoice?

I've got your back, @hazel-hlaing-orb. I'd be happy show you how you can add your company's registration number in your invoice in QuickBooks Online (QBO).

For us to add your company's registration number into your invoice, we'll need to create an invoice template. To begin, here's how:

1. Go to the Gear icon and then select Custom form styles.
2. Click New style and then Select Invoice.
3. In the Design column, add a name for your invoice template.
4. You'll then want to go to the Content column, and then scroll down to the display section to tick the box for GST Registration No.
5. Click the Preview PDF to get a glimpse of the invoice template that you've created.
6. Once you're good with your invoice template, click Done to save the template.

For visual reference, refer on the screenshot that I've attached below.

Now that we've created the invoice template, let's use the template in your invoice. To begin, here's how: 

1. On the left navigational bar, click the + New  button.
2. Under Customers column, select Invoice.
3. Choose the customer that you'd like to send the invoice and fill out the necessary details.
4. Click Customise and then choose the invoice template that you created earlier.
5. Once done, click Save and send.

Additionally, I've got you this article to help you manage invoice payments from customers in QuickBooks Online: Record invoice payments in QuickBooks Online.

 In case you'd like to add a discount to an invoice or sales receipt, feel free to visit this article for the steps: Add a discount to an invoice or sales receipt in QuickBooks Online.

It was nice to having you here in the Community, @hazel-hlaing-orb. If you need help performing specific tasks in QuickBooks Online, feel free to click the REPLY button below. Rest assured, we are always ready to assist you anytime. Thanks for choosing QuickBooks. Take care and have a nice day!