You should already be able to do this by using Customize IPTC Info
& Customize IPTC Stationery
in the Accessibility
section of PM's Preferences
More information can be read in PM's online manual
However, the default settings within the IPTC Info
is already 3 lines, and the default for IPTC Stationary
is 5 lines. If you are using Classic IPTC dialogs
(which only offers 1 line) you will need to uncheck that preference in order to be able to use the expanded IPTC dialogs and customise them.
Many many years ago I preferred the simple more compact Classic
dialog, especially on a small, low resolution laptop screen; but (and my memory may be fooling me here, but...) changes to IPTC/XMP standards resulted in Camerabits advising against using the Classic dialogs as they were deprecated and that the option would be removed from PM in the future (it hasn't so far) - so I reluctantly investigated the then new expanded dialogs; it only took a few minutes of customising and experimentation to configure a new superior layout that works really well for me.