Marking outgoing emails as important or high priority is something people do in the universe of Microsoft Outlook.
Gmail doesn't have priority flags like Outlook. But that's OK as many email clients ignore the flag.
Instead, Gmail learns which emails are important to you by evaluating several signals including who you send emails to frequently and which emails are opened quickly.
If the recipient is not expecting the email, consider following up the email by another channel (phone or chat) to ensure the email is opened in a timely manner.
Alternatively, consider adding to the start of the subject Urgent:, Important: or Action required: to draw attention to the message.
See your important emails in Gmail
Next to emails that Gmail thinks is important, you'll see a yellow Importance marker . If an email hasn't been marked as important, the marker will be empty.
In Gmail, you can mark received emails as "important" or with a star for better visibility.