How to enable Manual entry for Invoice Table in Subscriptions

By default, when trying to manually link an invoice to a Subscription, it will not be possible to do so since the field is usually read-only by default.

To change it to manual entry, you do the following:

1. Make sure developer mode is enabled and follow below screenshots.
2. From your subscription document, just click the three dotted button and select customize.

1. On clicking of customize the below screen will pop up, scroll down and in the 'Invoice' row, click edit to the far right.
2. 
3. In the below screen, uncheck the read-only checkbox and save your changes.