Multi Select Parameters in SSRS with default selection set to "Select All

In the case of Multi Select Parameters in SSRS,
How to set the default value to "Select All". I.e., by default, the Parameter value should be set to "SELECT ALL".

For example, if we have a @Region as a MultiSelect Parameter
with Values, "EA","SW",NW" and "HQ"

SSRS adds "Select ALL" check box to the Parameter list.

So We have Select All, EA,SW,NW,HQ check boxes appear in the Parameter List Box for the user to pick the Parameter Values for Region.

To start with,  the "Select ALL" check box should be checked(default selection)
How to achieve this ?