You need a specific thread adapter, such as the ASG Thread Adapter (18mm to 14mm) for Scorpion EVO 3A1, which converts the proprietary 18mm threading on the airsoft gun's barrel to the standard 14mm counter-clockwise (CCW) airsoft threading. 
 
 
Product Details
Function: This adapter replaces the original 18mm flash hider and allows for the attachment of standard airsoft accessories like barrel extensions, flash hiders, or suppressors that have a 14mm CCW thread.
Material: The ASG adapter is CNC-machined from aluminum for durability and has an anodized black finish.
Compatibility: It is specifically designed for the airsoft version, the CZ Scorpion EVO 3 A1, and ensures a proper fit and alignment.