Hi @Sheila,
Thank you for posting in Microsoft Q&A Forum.
Regarding to your description, I understand that you want to create a custom booking slot duration as the picture below:
If my understanding is correct, I'd suggest you follow the instructions below to perform the custom slot as your requirement.
1- Open the Calendar in New Outlook and Go to my booking page
2- Create a new booking. In the Meeting Type section, set the time to custom 20 mins
3- In the Schedule customization section, set your custom availability hours and expand the Advanced options
4- In the advanced option, set the Limit start time to: 20-min intervals. For minimum and maximum lead time, you can customize it as needed. In my example, I’ve set availability for just one week.
5- After setup, this is how the booking page will appear to others when they choose a time slot with you.
I hope this helps and thank you for your time. If you have any further questions or concerns, please don't hesitate to let me know.
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