Steps to schedule email in Outlook web version

Open the mail manager on the web and select New Message
Draft the message
Now hit the Dropdown icon next to Send at the bottom
From the available options, choose Schedule Send
Now, click on Custom Time
Now, just set the time and date and click on the Send button

If you want to know the detailed step by step process then read the blog:
https://datarecovee.com/how-to-schedule-an-email-in-outlook/