There isn't a way to force a value in the page header.  It reads the 'current' record of the dataset and then displays those values.  You could code a variable or you could change the order of the data so that the abc value is on top, but on other pages it might fail...the variable wouldn't change, but you need to read the data....you might try setting the value of the variable in the data filter section of the report, since it will read all the records trying to determine if a value should be included or excluded from the report.