Adding foreign key to existing table using Migrations

Hi,

Thanks for the great release of CI 4. Its wonderful. 
I recently started working with CI 4 over CI 3. I love the way migrations are setup. However I feel it lacks some functionality or not sure if I don't understand it.
I am trying to add a few foreign keys to an existing table. But I couldn't find anything in documentation except adding foreign keys while creating a new table. 

Can someone please point me into the right direction.

Thanks