To integrate Freshbooks with Freshservice, you first need to get the Freshbooks API & Authentication token. 

In Freshservice, 

1. Go to Admin → Apps → Get More Apps.

2. Locate the Freshbooks app and install it.

Log on to your Freshbooks account, click on your People tab and go on to the Staff and Contractors subtab. Click on Edit in your Admin, and you will be able to access your API URL and Authentication Token. 

Log on to your Freshservice portal & paste the API URL & the Authentication Token, update. Click on Enable.

Your Freshbooks account is now integrated with Freshservice.

You can now go on to your Freshservice portal, click on a ticket, go to the Time Sheet tab, and add your Time details, that can be then used to generate your invoice from Freshbooks.