Use Power Apps portals to create external-facing websites that allow users outside their organizations to sign in with a wide variety of identities, create and view data in Dataverse, or even browse content anonymously.
If you want to add a custom calendar picker in the power apps portal then you have to follow the below steps.
1. Create a webpage
2. Add a form on that page
3. Write custom JS or jQuery for the calendar
4. Add Validations (JS) as per your requirements
1. For creating the webpage, first you have to create a Web Template then a Page Template, and Web Page.
2. After creating the webpage, add a form or field for calendar picker.
3. When you have the field on the page then you can start to write custom js code for creating the calendar picker.
Below is some code reference for the custom calendar picker.
4. As your custom calendar is created now, then you can add the calendar validations as per your requirement.