Right-click the target message element in the Target pane of the Message Mapping editor and click Create
One-sided Mapping.
In the Overview view, the target appears in the Target column and
the word undefined appears in the Target Value column. Click this Target
Value to select it, then click the box that appears to the right of the value. The Compose Expression editor appears and you can assign the value
that you want to use for the target.
Click OK to save the mapping. The
value that you set for the target appears in the Target Value column.
An alternative method for adding a one-sided mapping is:
Select the mapping that you want to use.
Right-click the selected values then click Create
One-Sided Mapping.
In the Compose Expression editor, you can add mapping expressions
for sources and targets that have not yet been defined, but you cannot change
mapping targets.