Just in case someone may be doing a search one day....

Found a solution:

In the same click event of the OK button

AdhCognosRpt.Sections("Section2").Controls("Label5").Caption = cboChains.Text

where AdhCognosRpt is the name of the data report
and the sections refers to the page header (Section2)
on my report and "label5" is the name of the label whose contents I want to change....

cboChains.text is the selected client the user chooses to run the report for...[from a combo box]