How to do equivalent of npm install --save with dub?

[Andrew Grace via Digitalmars-d-learn]

I am trying to play with D, but I'm getting stuck with the DUB 
package manager.  If use DUB to download a package to my project, 
how do I get DUB to add what I downloaded to the dub.json file?  
I have tried DUB --fetch --cache=local http-parser (for example). 
 It downloads the package, but then nothing is added to the list 
of dependencies.

I'd appreciate any help.
Thanks,
Andrew