The DB Find Invoice for Credit Note form is accessed by clicking on the Find Invoice button
on the DB Credit Note form:

This form allows you to search for and select the required Invoice.
Add search criteria and click on the
button. A Grid will be displayed at the bottom of the form displaying the Invoices that meet the search criteria:

Select an Invoice by clicking on the link in the Invoice No column:

You will be directed back to the DB Credit Note form and the selected Invoice number will be added to the
field.