How to create a splash screen in perl and GTK3 for a flatpack program (gmusicbrowser)?

Bonjour à tous,

I just created (and published at Camel theme for gmusicbrowser - pling.com) a theme for the excellent open source audio player gmusicbrowser, and now, since it takes 6~7 seconds to load, I would like to complete my work with a splash screen, but I am not a developer.
gmusicbrowser is written in perl and uses GTK3. I have read and tested the scripts offered at firefox - How to create a custom splash screen for a program? - Ask Ubuntu, but not only do they do not work as is (on ubuntu 24.10, GNOME Shell, ubuntu on Xorg), if Firefox (affected by these scripts) is not already open, but they do not work at all with gmusicbrowser installed by flatpack (its command is /usr/bin/flatpak run --branch=stable --arch=x86_64 --command=gmusicbrowser --file-forwarding org.gmusicbrowser.gmusicbrowser), which would be best, not even with gmusicbrowser installed by .deb. Paolo Molaro wrote a tutorial (404 Not Found) but it is very old.

Thanks for a helping hand!