When you apply custom masking to a field, click

Configure

and enter the expression. You can select the source fields, operators, and functions to build an expression. When you select a function, you can view the function description and the syntax.

You can concatenate data from multiple source fields to create a masked value for the target field. For example, you need to create a login name. The source has FirstName and LastName fields. Use substitution masking to mask the first and last names. In the Login field, configure an expression to concatenate the first letter of the first name with the last name:

CONCAT(SUBSTR(FirstName,1,1),LastName)

To mask field input values with null values, use custom masking. In the expression builder, enter single quotes separated by a space in the following format:

' '

For more information about configuring expressions, see Field expressions