When a web user indicates that they are an existing customer you can help your team by prompting the web user to provide their custom reference number or other information that may be useful.
Create this rule on the Customer Reference Number Field.
Trigger:
As this is a field the web user will interact with we will set this rule to run continuously.
Conditions (IF):
There may be several ways of identifying whether a web user is an existing customer. There may be a simple checkbox with the label "I am an exiting customer" but this would also work for radio buttons or gate editor fields.
In this example we will configure the rule to run if the ExistingCustomer field is checked.
Actions (THEN):
Show this field, otherwise hide this field.