When the Case Mapping transform is selected, the General tab in the Properties view allows you to specify any valid ESQL expression.
To specify values for a Custom Mapping transform:
Verify that the type of the transform that is selected in the mapping routine area is Custom Mapping.
In the Properties view, select the General tab.
In the General tab, in the Custom mapping expression input field, type the ESQL expression that you want to be evaluated.
At runtime:
The runtime environment evaluates the custom mapping expression and obtains a resulting value.
The runtime environment stores the resulting value into the target element.