I would try:

    public boolean isSelfServicePortalServerAvailable(String crmEndPoint) {
        boolean isServerAvailable = false;
        try {
            HttpURLConnection connection = getHttpURLConnection(crmEndPoint);
            connection.connect();
            isServerAvailable = true;
            connection.disconnect();
        } catch (MalformedURLException e) {
            log.error(e.getMessage());
        } catch (IOException e) {
            log.error(e.getMessage());
        }
        return isServerAvailable;
    }

    private HttpURLConnection getHttpURLConnection(String crmEndPoint) throws IOException {
        URL url = new URL(crmEndPoint);
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        return connection;
    }

And then test

    @Mock
    private HttpURLConnection mockConnection;

    @Mock
    private ISelfServiceDao selfServiceDao;

    @Mock
    private RestTemplate restTemplate;

    @InjectMocks
    @Spy
    private SelfServiceCoreClaimImpl selfServiceCoreClaimImpl;



    @Test
    public void testIsSelfServicePortalServerAvailable_Success() throws IOException {
        String crmEndPoint = "http://test-url.com";
        doReturn(mockConnection).when(selfServiceCoreClaimImpl).getHttpURLConnection(crmEndPoint);
        doNothing().when(mockConnection).connect();
        doNothing().when(mockConnection).disconnect();

        boolean result = selfServiceCoreClaimImpl.isSelfServicePortalServerAvailable(crmEndPoint);
        assertTrue(result);
    }

You can’t just inject mock for objects that are created inside the method (like the new URL(crmEndPoint);).

The @InjectMocks will automatically inject the mocks inside private fields, or setters or constructor parameters.

If an object is created inside a method then you have to consider the following:

1. Is this object a stateless? Then I have to receive it from the constructor or DI system and then mock it. So I have to refactor the method and remove the creation of the new object.
2. Is this object a DTO? Then I don’t need to mock it.
3. If none of the above, which means the object creation can’t be avoided, (probably your case as I can tell) then you have to use the MockedConstruction.
   Also for the mocked construction to work you will need the following dependency in Maven:

<dependency>
    <groupId>org.mockito</groupId>
    <artifactId>mockito-inline</artifactId>
    <version>some version here</version>
    <scope>test</scope>
</dependency>

I’ve done few tweaking in your junit, and it runs perfectly fine.

    @Test
    public void testIsSelfServicePortalServerAvailable_Success() throws IOException {
        HttpURLConnection mockConnection = Mockito.mock(HttpURLConnection.class);
        URL mockUrl = Mockito.mock(URL.class);
        SelfServiceCoreClaimImpl selfServiceCoreClaimImpl= new SelfServiceCoreClaimImpl();
        Mockito.doReturn(mockConnection).when(mockUrl).openConnection();
        Mockito.doNothing().when(mockConnection).connect();
        Mockito.doNothing().when(mockConnection).disconnect();

        boolean result = selfServiceCoreClaimImpl.isSelfServicePortalServerAvailable("http://test-url.com");
        assertTrue(result);
    }