Is ORDER_EXTERNAL_ID a user fillable field?

 
I noticed the ORDER_EXTERNAL_ID in the order properties docs. Docs say: 

Order identifier in an external trading system (on the Exchange)

Does that mean external as in we can input an ID there? If so how can we set this value?

This would be helpful to track and group orders that belong to a trade in your strategy, like scale in and scale out orders - especially for netting accounts where everything is consolidated into one position.

 
I guess (never checked) it is the ID of the market were it is traded, other then the ID from the broker server (=ticket number) or your ID (=Magic Number).
 
dazamate: I noticed the ORDER_EXTERNAL_ID in the order properties docs. Docs say: Does that mean external as in we can input an ID there? If so how can we set this value?
There is no "OrderSetString()" function, meaning that order properties, like ORDER_EXTERNAL_ID are read-only. You cannot set them.
Reason: