SetEnv API_USERNAME 'my_special_username'
SetEnv API_PASSWORD 'my_special_password'

echo $_SERVER['API_USERNAME'];
echo $_SERVER['API_PASSWORD'];
// or
echo getenv('API_USERNAME');
echo getenv('API_PASSWORD');

[Service]
Environment="API_USERNAME=my_special_username"
Environment="API_PASSWORD=my_special_password"

echo getenv('API_USERNAME');
echo getenv('API_PASSWORD');
// $_SERVER does not work when I specify environment variables in systemd
echo $_SERVER['API_USERNAME']; // returns blank
echo $_SERVER['API_PASSWORD']; // returns blank