How to implement 'Starts With' for exposed filter of User ID
If you use the "User: Name" filter it will give you an autocomplete text field where you can start typing and it will show you a list of patching users as you type.

That field will not let you enter partial user names though, you must select a user from the autocomplete (you can have multiple comma delimited values if you like).

To match partial user names, use the "User: Name (raw)" filter.

This one allows you to have a text field that doesn't have any validation.
This field can have the operator set to "Starts with".