Depends a bit on your domain model. But in general:
You will need an email address to send the email to. If the selected person from the drop-down is an entity that also has an email address attribute, you can use that to send the email.
In the submit microflow you can retrieve the account by association from your context entity. Then you have your selected person. Then you use the functionality from the email connector module(https://marketplace.mendix.com/link/component/120739) to send your email, you use the emailadress from the person entity to do so.
Hope this helps