Assuming you want to test this class

public class MainService {

	private MyService myService;
		
	public String doStuff(String value) {
		return myService.doStuff(value);
	}

}

where MyService has the following implementation

public class MyService {

	public String doStuff(String value) {
		return "some string";
	}
}

If you want mock method doStuff of class MyService to return the same value of the input parameter you can do the following:

import static org.junit.Assert.assertEquals;
import static org.mockito.ArgumentMatchers.anyString;
import static org.mockito.Mockito.when;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.junit.MockitoJUnitRunner;

import com.aptar.mes.service.MainService;
import com.aptar.mes.service.MyService;

@RunWith(MockitoJUnitRunner.class)
public class SpringTestApplicationTests {

	@InjectMocks
	private MainService mainService;

	@Mock
	private MyService myService;

	@Test
	public void doStuffTest() throws Exception {
		when(myService.doStuff(anyString())).thenAnswer(invocation -> { return invocation.getArgument(0); });
		assertEquals("hello", mainService.doStuff("hello"));
	}
}

Answer is used when you need to do additional actions when a mocked method is invoked, e.g. when you need to compute the return value based on the parameters of this method call.
Below an example of test