Welcome to the Community, @suddin-cinnova-c.
I'd love to assist you promptly here in the Community, however, we have a separate online forum called Developer Community. This is where our developers interact with others to talk about programming concerns. I suggest you ask there about the correct mapping of the custom field in your invoice using the .NET SDK, so you'll be given the proper guidance.
Here's how:
- Go to this link: Intuit Developer Support
- Log in to your Intuit Developer Account or Create a new one
- Below the What you need help with, choose QuickBooks Online.
- Click Ask a question
- Hit Ask
Additionally, I added this link that can help you learn how to personalize your invoice: Customize invoices, estimates, and sales receipts in QuickBooks Online
You can always get back to us whenever you have additional managing invoices or any QuickBooks-related questions. Stay safe.