Ensure that the Business Logic Subject's parameter is set to Mandatory and enter data in the "Prompt user for a parameter value" section. Take note of the Required Value Name. In the example below this would be "Enter Department".
When setting up the Query Condition for the criteria to join the parent data to the parameter value in the Busienss Logic Subject the drop down list of available fields will display the required value parameter. The value "Enter Department" is highlighted in the image below. This is the value to select for the field in the child subject.
The matching value should be set in the parent query. In the image below this would be "Dept Code". What the query runs it will pass the value of the parent's "Dept Code" into the Business Logic Subjects Required Value Parameter, "Enter Department" and so bring data back for each parent record.
A sample report is below showing the first two columns of data retuned from a Free-from subject ( Department), and the remaining columns being returned to calls to the Business Logic Subject (EmployeeBL).