`panel_custom` not working as expected for links to other device UIs

Hello!

I wanted to add an extra menu entry in HAOS’ sidebar to easily reach my other FHEM installation’s WebGUI at:
http://fhem.fritz.box:8083

Here is my code snipped from configuration.yaml from HAOS at http://ha.fritz.box:8123:

panel_custom:
  - name: fhem_panel
    sidebar_title: "FHEM"
    sidebar_icon: mdi:home-assistant
    js_url: /api/hassio/app/entrypoint.js
    url_path: "fhem_ui"
    embed_iframe: true
    require_admin: true
    config:
      ingress: false
      url: "http://fhem.fritz.box:8083"

When hovering over the new sidebar entry of “FHEM” and clicking it, I get this (wrong) URL:
http://ha.fritz.box:8123/fhem_ui

I have studied these links to no avail:

What am I missing? Thanks for your help!

EDIT:
It works when going to Settings > Dashboards > Add dashboard > Website, and entering http://fhem.fritz.box:8083

Why the panel_custom way does not work for me, and what I did wrong with it, I am still interested to learn. :blush: