I believe ASSETID is part of AST application and not CMDB, correct?
You will need an extra push to AST:Attributes to push the ASSETID.
Since Eduardo is using Spoon, he may not understand what you mean by "push" -- one way to do this in Spoon is by connecting output of CMDB output action to input of AR upsert action with AST:LoadAttributes set as target form and instance ID from first action mapped to instance ID of targeted form. That would emulate BMC's process (and require validation and promotion); alternatively, you could target AST:Attributes if you don't mind writing directly into production database.
More details about a similar job can be found in Importing Assets using Spoon - connection missing between forms.