To add tasks to Marvin using your voice, you'll need to install the Automate app from Google Play Store.

Once you're in the app, open the sidebar and tap on the "Community" link. This will open the list of user-submitted automations that you can download and use. Open Search on the top-right and type "Marvin". You will see one called "Add tasks to Amazing Marvin" with #36402 ID. Open it and click on the "Download" button.

In order for tasks to end up in your Marvin, you'll need to edit the flow and add your API token. First, open Marvin on your laptop/PC and go to ☰ → Strategies → API. Click on the "View Credentials" button and copy your apiToken:

Save this token somewhere and go back to Automate. Open the "Add tasks to Amazing Marvin" flow and click on the edit button on the bottom right. This will open the flow which contains blocks that are used to automate this process. Tap on the "Set variable headers..." block and paste your apiToken where the "Your-API-Token-Goes-Here" text is and keep the quotation marks.

To easily access this flow, add a shortcut or a widget for it to your home screen. You'll be asked to choose which flow you want to add and to choose an icon.