Add a $filter path segment that specifies a Boolean expression, where the expression is specified through a query parameter:
POST Products/$filter=@filter/Special.Discount?@filter=Age gt 3
This can be used in conjunction with an action, a data modification statement, or any other statement.
This follows the existing semantics that, if the segment following a collection matches an OData segment, that segment takes precedence over a possible key value in key-as-segment.