Thanks for posting in the Community, Joanna.

Yes, we can add the link/URL link to the email message prior to sending the invoice to your customer. 

If it is a one time add on to the message body:

1. Click on the Sales tab.
2. Click on the Invoice and click on Edit.
3. Click on Save and Send.
4. From the email message, enter the link/URL link you wanted to add.
5. Click on Send and close.

If you wanted to permanently add the link:

1. Click on the Settings icon.
2. Click on Account & Settings.
3. Click on Sales.
4. Click on Messages and enter the link.
5. Click on Save and click on Done.

That's it. For reference you can also check this link to learn more about: How to set up or change customer messages in QuickBooks Online. 

Post again in the Community if you have further questions. I am around to help you.