One of the great features of Todoist is the capability to hit Command-Control-A (Or a key combo of your choosing) anywhere/anytime and up pops a window for me to enter.a quick task and assign it to a project. This is especially good for when I am in email and want to create a to-do item from that email.
It would be nice to have a similar thing for Agenda…hit a key combination of your choice, and up pops a window for you to enter a note and assign that to a project. That would be a good answer to integration, for the short run, The only other answer would be to create a service that’s available in other apps.