<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.0.0.RELEASE</version>
</parent>

@Configuration
@PropertySource("file:/etc/koshka/application.properties")
public class Config
{
    @Autowired
    private Environment env;

    @Bean
    public static PropertySourcesPlaceholderConfigurer configurer() {
        return new PropertySourcesPlaceholderConfigurer();
    }

    @Value("${socmess.pass}")
    private String pass;

    public String getUser() {
        return env.getProperty("socmess.user");
    }

    public String getPass() {
        return pass;
    }
}

$ cat /etc/koshka/application.properties
socmess.user="testuser"
socmess.pass="testpass"

@RestController
public class Sender {
    private final Logger logger;

    public Sender()
    {
        logger = (Logger)LogManager.getLogger("KoshkaBot");
    }

    @GetMapping("/vk")
    @CrossOrigin(origins = "*")
    public ResponseEntity<String> vk(
        @RequestParam(value="username") String username,
        @RequestParam(value="password") String password
    ) {
        Config conf = new Config();

        logger.info(conf.getUser());
        logger.info(conf.getPass());

        return ResponseEntity.ok().body("OK");
    }
}