Hi Ayush,
To set the default selected values to the MultiSelect dropdown widget, you will have to explicitly set the IsSelected attribute of all the default records to a True value.
You can assign the IsSelected attribute with boolean value (conditionally, either True or False) in the DataSource action or in OnAfterFetch event handler action flow.

Demo Screen: MultiSelectTask
Refer to the attached oml.
I hope this helps you!
Kind regards,
Benjith Sam