What is the command for Package Manager | Settings | Repositories | Source code

What is the command for Package Manager | Settings | Repositories | Source
code

I tried to do step two of building Banshee from source, but it failed as
follows:
~$ sudo apt-get build-dep banshee
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to find a source package for banshee
I was able to solve this problem by executing Package Manager | Settings |
Repositories | Source code, followed by the command
sudo apt-get update
After which the build-dep step succeeded.
Is there a non-GUI way of enabling the source code repositories for
apt-get? The man page says there's apt-get source, but it isn't clear to
me that command has the same effect.