Since April 9, 2019 the pre-check has been removed completely by Facebook, so there's no option to have the widget pre-checked at the moment.
Even though it was really easy to collect Messenger subscribers with a pre-checked Messenger Widget, it also led to a negative user experience as many people were not even aware that they subscribed. 😕
We've been closely working with Facebook for a long time now and being an official Facebook Marketing Partner, we strongly support their decision.
Thanks to their help, we’ve been able to come up with several alternatives for the pre-check.
Conversation Starters have been tested and have proven to be a great way to get subscribers. Being long-term oriented, these tools will contribute to you building a better merchant-customer relationship. 👌