Q:

How do I implement and return a Cmd Msg? 

How do I implement and return a Cmd Msg? 
For example, the following line generates a Cmd Msg:
Http.send msg request

It's used in the following function:
tryRegister : Form -> (Result Http. Error JsonProfile -> msg) -> Cmd msg
tryRegister form msg =
    let
        registerUrl =
            "http://localhost:5000/register"

        body =
            encode form |> Http.jsonBody

        request =
            Http.post registerUrl body decoder
    in
        Http.send msg request

I'm trying to hand code a similar function within my TestAPI:
tryRegister : Form -> (Result Http. Error JsonProfile -> msg) -> Cmd msg
tryRegister form msg =
    Cmd.none

The above code compiles.  However, it's not clear to me how to implement a function that returns a Cmd Msg other than Cmd.none. 
Appendix:
type Msg
    =
    ...
    | Submit
    | Response (Result Http. Error JsonProfile)

update : Msg -> Form -> ( Form, Cmd Msg )
update msg model =
    case msg of
       ...
       Submit ->
           ( model, runtime.tryRegister model Response )

Source code on GitHub. 

A:

Edit
The original answer suggested mapping over Cmd.none, which compiles and may potentially be useful when mocking out functions for testing, but if you are actually trying to force another update cycle in The Elm Architecture, you will need to convert to a Task, as outlined in the send function described here. 
send : msg -> Cmd msg
send msg =
    Task.succeed msg
        |> Task.perform identity

As @SwiftsNamesake mentioned above, in most cases this is not necessary, and the entire blog post on the subject is worth a read. 
Original Answer
You can use Cmd.map over Cmd.none to change it to any Cmd:
Cmd.map (always Submit) Cmd.none

