To add high priority to an email, press the 3-dot menu in the compose screen and choose Set High Priority.
You can also set this option as default by tapping More (o o o) | Various Account Settings | tap the account | Advanced Account Settings | Use High Importance.
TypeApp marks high priority emails with a blue circle and an up arrow within.
Email has a priority header and this sets it to high. Some clients (e.g. outlook) show special markers (e.g. !) to high importance emails.