You can install the schema by executing the following commands on a Linux or macOS machine:
mkdir -p $HOME/.local/share/glib-2.0/schemas
cp ./src/schemas/net.rust-gtk4-boilerplate.gschema.xml $HOME/.local/share/glib-2.0/schemas/
glib-compile-schemas $HOME/.local/share/glib-2.0/schemas/
On Windows run:
mkdir C:\ProgramData\glib-2.0\schemas\
cp .\src\schemas\net.rust-gtk4-boilerplate.gschema.xml C:\ProgramData\glib-2.0\schemas\
glib-compile-schemas C:\ProgramData\glib-2.0\schemas\
Run:
cargo run --bin gtk4_application