Launch Signal if not running, otherwise toggle hidden/shown state. Put signal.desktop in ~/.config/autostart/.

signal

#!/bin/bash

run () {
    signal-desktop &
    disown
}

show () {
    local -r id="$1"
    idx="$(printf "0x%08x" "${id}")"
    wmctrl -i -r "${idx}" -b remove,skip_taskbar,skip_pager
    wmctrl -i -R "${idx}"   # really make sure it's raised
    xdotool windowraise "${id}"
}

hide () {
    local -r id="$1"
    idx="$(printf "0x%08x" "${id}")"
    wmctrl -i -r "${idx}" -b add,skip_taskbar,skip_pager
    xdotool windowminimize "${id}"
}

get_id () {
    xdotool search --name '^Signal( \([0-9]+\))?$'
}

missing=false
if ! command -V wmctrl >/dev/null; then
    echo "Please install wmctrl"
    missing=true
fi
if ! command -V xdotool >/dev/null; then
    echo "Please install xdotool"
    missing=true
fi
if ! command -V xprop >/dev/null; then
    echo "Please install x11-utils"
    missing=true
fi
if "${missing}"; then exit 1; fi

if ! id="$(get_id)"; then
    run
    if [[ $1 = hide ]]; then
        for i in {1..20}; do
            if id="$(get_id)"; then hide "${id}"; break; fi
            sleep 0.5
        done
    fi
else
    if xprop -id "${id}" | grep -q '^_NET_WM_STATE(ATOM) =.* _NET_WM_STATE_SKIP_PAGER'; then
        show "${id}"
    else
        hide "${id}"
    fi
fi

signal.desktop

[Desktop Entry]
Encoding=UTF-8
Version=0.1.2
Type=Application
Name=Start Signal minimized or toggle
Comment=Start Signal Private Messenger and immediately hide it, or toggle if already running
Exec=/path/to/above/signal hide
StartupNotify=false
Terminal=false
Hidden=false

toggle-signal.gif