How to allow an argument AND no argument and act differently on them with getopts

I want to have getopts setup for my script in such a way that it accepts no argument (-p) but so that it can also accept an argument (-p library). Both need to be accepted. They're (argument and no argument) are both allowed. The argument should be allowed to be any string. See below:

while getopts "p:sdih" opt; do
                case ${opt} in
                        p )
                                #WHEN NO ARGUMENT IS GIVEN, DO THIS

                                selection="plex"

                                #WHEN AN ARGUMENT IS GIVEN, DO THIS

                                selection="plex"
                                argument=true
                                optarg="$OPTARG"
                                ;;

                        s )
                                selection="sonarr"
                                ;;

                        d )
                                selection="deluge"
                                ;;
                        i )
                                selection="info"
                                ;;

                        h )
                                usage | column -t -s "|"
                                exit
                                ;;
                        \? )
                                echo "Invalid option"
                                usage | column -t -s "|"
                                exit
                                ;;
                esac
        done 2>/dev/null
        shift $((OPTIND -1))

That's how it needs to work. But I'm new to getopts so I don't know how to do this yet...