-- Some trigger/program unit text
Declare
  ....
Begin
  ...
  :your_block.your_item := 'ENTER SOME TEXT HERE.';
  ...
End;

Begin
  -- If the checkbox is checked, put a text in the text item
  If CheckBox_Checked('my_block.chk_ok') Then
    :myblock.txt_message := 'checkbox is checked';
  Else -- put another text in the text item
    :myblock.txt_message := 'The checkbox is not checked now.';
  End if;
End;