On a branded short domain you can have your links generate with an https by default.
If you're using the bit.ly domain you would need to add it manually to make the link secure.
After copying your link, add the "s" after the "http"
I copied http://bit.ly/2I7jQIQ
Before I send it I add in the "s" making it https://bit.ly/2I7jQIQ