Preselect or set a default value

## What

I would like to know how I can preselect/prepopulate a value in autocomplete input text. If that is not possible is there any way to set a default value? HTML value="default" does not work.
## Use case

I am trying to use it in an email composer's to, cc, bcc fields. By default the input fields have some default user selected, ( it may be different depending on case) but they can change it using the search in autocomplete.

Start by examining the autocomplete input API and its existing handling of initial values. Determine how a preselected or default value should work for the email composer use case, including whether users can replace it through autocomplete. Done means the documented behavior is supported and covered by the relevant existing tests, if present.

Written by the indexing model from the issue text.