Hi Alvin2818,
Thank you for posting. It's not feasible to send message mark as a high importance using Outlook for iOS.
If you have any further query you may please use built-in app support to contact our dedicated Outlook app for iOS team for further assistance.
In the Outlook app, Tap your profile photo and then the ? > Contact Support. For your reference: Get in-app help for Outlook for iOS and Android
NOTE: When you contact our support team, you won't receive an email or get a call back. But you'll receive a notification when the support team sends you a reply and can easily communicate with them in the app. Look for the notification on the Settings and Contact Support icons.
Your understanding is highly appreciated.
Best regards,
Shyamal