Where I have to write TRN number while adding the supplier ?

Welcome to the Community, @accounts-sapient.

I'll help you ad  TRN when entering the supplier. I can guide you on how to do it.
 

1. Go the Gear icon.
2. Choose Custom Form Styles.
3. Select the Invoice template you're using.
4. From the Actions column, click Edit.
5. Pick the Content tab.
6. Select the Header part on the sample invoice.
7. Click the Company registration number link. 
8. Enter the company tax number in the CRN field.
9. Click Done.
    

You can read through this article for more detailed information about personalizing your sales forms: Customise invoices, estimates, and sales receipts in QuickBooks Online.

Also, I've added these articles to learn more ways to manage your sales forms in QuickBooks:

• Add, customise, or remove logos on sales forms
• Customise sales forms by location
   

Always feel free to leave a comment below if you have other questions. I'm always here to lend a hand.