Installing
2. Setting Up Flatpak
How to Install Flatpak on Slackware
3. Installing Signal Messenger
Then to Set up Signal App on Slackware
Simply play:flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
For further Insight see the Flatpak Official Documentation.
And after:flatpak install https://dl.flathub.org/repo/appstream/org.signal.Signal.flatpakref
The Flatpak App Setup do Not need the sudo Command because then it Runs confined into a Sandbox.
However you’ll be Prompted to Authenticate with your User or Admin Pass.
Then Confirm the proposed Setup with ‘Enter’ and wait until the App is Downloaded and Installed.4. Launching Signal Desktop
Finally, Launch & Enjoy Signal Messenger ;)
Make Use of the Desktop Launcher:Or from Shell run:
flatpak run org.signal.Signal
Over the First, Start you’ll be Prompted to Link your Mobile Device by the QR code:
Just follow the Instructions reported on Screen…

So Now I’m truly Happy if this Info could Help you to Install Signal App on Slackware!