Hi @derk,
To send a SharePoint calendar event to an external email address, we could try to configure the flow likes below.
Say you are using the trigger “SharePoint – When an item is created”, in the List Name field, enter the calendar name manually. Then add the Send an email action, in the To field, enter the email address for external users, fill the Subject and Body fields with related information.
When a new event is created, an email will be sent out to the user you have specified.
Please take the following screenshot for a reference.

Best regards,
Mabel Mao