Answer:
You can't.
Step-by-step explanation:
I don't know why they still have that feature (they claim to be removing it soon), but it's a way to get you to use Brainly more.
By saying you need to reach a specific goal to be rewarded with a messaging system is a way to get people to actively use Brainly.
Looks like you'll have to stick with the comment sections like the rest of us.
