You can add tasks to Marvin with Siri. There's nothing to set up.
Adding a task
Say one of:
"Add a task to Marvin"
"Add to Marvin"
"New Marvin task"
Siri asks what you'd like to add. Say the task. You can include a date, a duration or #category ("Call the bank tomorrow"), or say a list: "...to my Errands list". New tasks go to your Inbox unless you say otherwise.
Siri confirms: "Added ... to ...".
Brain dumping to Siri
Say "Brain dump to Marvin". Siri asks what's on your mind. Say everything, and Marvin splits it into separate tasks.
Shortcuts app
The same two actions, Add Task and Brain Dump, are in the Shortcuts app, so you can use them in your own shortcuts and automations.
Screenshot to add: Marvin actions in the Shortcuts app