Icon for self build application not showing in window list (hover over icon)

[simmihugs]

In the window list a placeholder icon appears

In docklike (in the panel) itself, the correct icon appears, in alt-tab it is correct

I'm using Xubuntu 22.04 xfce 4.16

The version of docklike-plugin I used to build

╭[simmi@xubu] ~/Projects/docklike-plugin
╰─> git log -1                                                on branch master
commit fe9c96cdfee0399723e8d696b561d01d9cd65fd2 (HEAD -> master, origin/master,
origin/HEAD)
Merge: 4113ad8 62473a5
Author: [nico@bach ~]$ ▮ <[email protected]>
Date:   Sun Jan 23 02:27:11 2022 +0100

    Merge pull request #125 from tgerov/patch-1

    Add Fedora RPMs in README
╭[simmi@xubu] ~/Projects/docklike-plugin
╰─>

What I did and what does work:

I build st terminal, and installed it into /usr/local/bin/st
additionally I created an st.desktop file at /usr/share/applications/st.desktop
with the content

[Desktop Entry]
Type=Application
Exec=st -e zsh
TryExec=st
Icon=org.xfce.terminal
Terminal=false
Categories=System;TerminalEmulator;

Name=st
GenericName=Terminal
Comment=st is a simple terminal implementation for X
StartupWMClass=st-256color

With Icon=org.xfce.terminal the whiskermenu shows an icon for st

and after copying an icon into place sudo cp /usr/share/icons/hicolor/48x48/apps/org.xfce.terminal.png /usr/share/pixmaps/st-256.png
and StartupWMClass=st-256color docklike taskbar is showing an icon in the taskbar as well