There are two ways to do this...

At the command-line, enter:

  $ hpterm -sb -sl 1000 &

The second way is to execute the following series of commands, prior to
running hpterm:

  $ xrdb -m
  HPterm*scrollBar: True
  HPterm*saveLines: 1000
  (Ctrl-D)

You can set saveLines to 1000 or some other arbitrarily large number.
Alternatively, you can set saveLines to something like "4s", which
indicates four screens.  You can also set the above resources in an
app-default file (/usr/lib/X11/app-defaults/HPterm).