Add option to fill in branch name with current branch name when branching off

When I create a new branch I very often use the existing branch as a prefix and add something to the name, e.g. `implement-feature0815` --> `implement-feature0815-specs`

I would like to have an option to make the branch name field for the new branch to be filled in with the current branch's name.

I tried copy-pasting the old name into the field but that does not work because selecting with the mouse is disabled and it would not be an ideal solution for a CLI app.