Fantastical comes with extensions for macOS: a Today widget for displaying the current day's events and tasks, and Share and Action widgets for sending text to Fantastical from your favorite apps.
Make sure that the Fantastical extensions are enabled by opening System Preferences and navigating to the Extensions pane. Select All on the left, scroll to "Fantastical" in the list on the right, and check the settings for "Share Menu," "Today," and "Actions."
To enable the Today widget, reveal Notification Center and click the "Edit" button at the bottom. Click the "+" button next to Fantastical in the list on the right.
The Action extension allows you to send snippets of text from compatible apps. Select the text you'd like to send and hover over it with your mouse cursor.
A dotted outline will appear around the text, with a down arrow in the bottom-right corner. Click the arrow and select "Add to Fantastical."
The Share Extension is available anywhere you see the Share icon in the toolbar. Click the icon and select "Add to Fantastical" from the popup menu.
Fantastical includes a system service, accessible from the contextual menu that appears whenever you right click on selected text. You can use it in almost any app to send text you're reading directly to Fantastical.
Just select the text you want to send, right click (⌃-click) and choose "Add to Fantastical" from the Services listed in the menu. Fantastical will open in your menu bar with the selected text parsed and waiting in the entry field. You can edit it as needed before clicking "Add Event" or pressing Return to create the event.