How to mock inject a config value in test class

I have an injection like

@Inject
    @ConfigProperty("BEHOLD_UDSLUS_BEREGNING_MND")
    private String maxMonths;

Now i want to mock this in my test class so that the value of "60" can be passed onto the main method. I tried using ReflectionTestUtils.setfield() but somehow its not getting mocked.

List<UdbKonfiguration> udbKonfigList = new ArrayList<UdbKonfiguration>();
        UdbKonfiguration udbKonfig = new UdbKonfiguration();
        udbKonfig.setKode("BEHOLD_UDSLUS_BEREGNING_MND");
        udbKonfig.setValue("60");
        udbKonfigList.add(udbKonfig);
        try {
            ReflectionTestUtils.setField(udbKonfig, "BEHOLD_UDSLUS_BEREGNING_MND", "60", null);
        } catch (Exception e1) {
            // TODO Auto-generated catch block
            e1.printStackTrace();
        }