How To link Header Text part of an Sales order in time of Sales order Print

Dear Guru,

Here I have encountered an Issue in time of Sales order printing.

As per our requirement I have to print the "Header Text" details (eg: Special Instruction)" of an Sales order in Order printout form.

But unlike any other field I am not able to find out the table and table fields in case of Header text.

So guru I want to know how i should link that in to my print program so that what ever instruction you are giving in "Special Instruction field " in header text of an Sales order would come into my print program's output.