This mapping defines a literal (constant value) to add to a message. To add a literal, the mapping should only have a "to" field that defines the field name and a "Replacement string" field that defines the field value. The mapped message will have this extra field. To use a literal mapping, set the function type to String functions and the function to "Add a literal value".
This example shows a literal mapping that adds an "monitoringSource" field to the mapped message with the value "1Gateway".