I have tried all the apps that integrate with RTM on the Windows desktop. Some were OK but one very important thing was always missing: Smart Add. At some point, I decided to just use the web app, but bringing the website up to add a task has considerable friction.

My solution: Google Chrome app + AutoHotkey

I have the following in my AutoHotkey script file:
;--------------------------------------------------
^!i::
IfWinExist, Remember The Milk - Tâches de Carl
{
WinActivate ; Automatically uses the window found above.
return
}
else
Run "C:\Users\cbolduc\AppData\Local\Google\Chrome\Application\chrome.exe" --app="https://www.rememberthemilk.com/home/carlbolduc/#section.tasks"
return
;--------------------------------------------------

Whenever I type Ctrl+Alt+i, the RTM site comes up in a naked Google Chrome window. From there, I can just type "t" to focus the Smart Add box and start typing.