Let's make sure this will added to the next upcoming updates, Trumpbox.

QuickBooks Online (QBO) integrates with different third-party applications to add powerful functionality to your accounting processes. 

For now, adding your own payment link to the invoice isn't an option. You can search and use a third-party application instead. Search one from the Apps menu, then go to the Find apps. 

Another option is to go to this link to search for an application.

Moreover, learn how the customer pays you so you can create and send an invoice. For more details, I've added this article: Record Invoice Payments In QuickBooks Online.

Let me know if you have any questions about payment link in QuickBooks. I'm always one post away if you need further assistance.