name - the name of the library. Library names must contain only basic letters (A-Z or a-z) and numbers (0-9), spaces ( ), underscores (_), dots (.) and dashes (-). They must start with a letter or number. They must contain at least one letter.
As the name hints, the "Additional Boards Manager URLs" field of the Arduino IDE's preferences is for the Boards Manager feature that installs Arduino boards platforms. It can not be used for Library Manager, which installs Arduino libraries. Note that Arduino libraries and Arduino boards platforms are two different things.
Use the correct metadata file name..
Make the metadata specification-compliant.
Provide detailed installation instructions in the library's readme.
The Arduino Library Manager is for installing and updating libraries that are listed in the Arduino Library Manager index. It simply isn't meant for that purpose. Arduino CLI does have such a capability:
(currently not working for your specific library due to a bug)
But it's easy enough to just use GitHub's Code > Download ZIP button to download a .zip file of the repository and then use the Arduino IDE's Sketch > Include Library > Add .ZIP Library to install it (also supported via arduino-cli lib install --zip-path).