According to the screenshot, it is the default reminder setting on Outlook Web.
There is no way to set a custom default reminder time on Outlook web. To get this feature more quickly, I have found a post in feedback collection website, and you could vote for it. Many features of our current products are designed and upgraded based on customers’ feedback. You can also make suggestions on that website and hope Microsoft will notice this in the future. Thank you for your understanding and support!
To set default reminders in Outlook client, please go to File>Option>Calendar>Calendar option>Default reminders.
Also, based on this problem, we have conducted many tests, the default reminder time also depends on the time of setting the meeting, if the setting time and meeting time is close, a reminder pop-up window will appear immediately.
Hope this helps!
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