Q:

Add marker on long press in Google Maps API v3

How to implement adding a marker on long press in Google Maps API v3 for android? There are answers about this in Stack Overflow itself but they are for Google Maps API v2.
public class advertiserMap extends FragmentActivity implements OnMapReadyCallback {

private GoogleMap mMap;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_advertiser_map);
    // Obtain the SupportMapFragment and get notified when the map is ready to be used.
    SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager()
            .findFragmentById(R.id.map);
    mapFragment.getMapAsync(this);
}

//for searching for a location
public void onMapSearch(View view) {
    EditText locationSearch = (EditText) findViewById(R.id.editText);
    String location = locationSearch.getText().toString();
    List<Address> addressList = null;

    if (location != null || !location.equals("")) {
        Geocoder geocoder = new Geocoder(this);
        try {
            addressList = geocoder.getFromLocationName(location, 1);

        } catch (IOException e) {
            e.printStackTrace();
        }
        Address address = addressList.get(0);
        LatLng latLng = new LatLng(address.getLatitude(), address.getLongitude());
        mMap.animateCamera(CameraUpdateFactory.newLatLng(latLng));
    }
}

@Override
public void onMapReady(GoogleMap googleMap) {
    mMap = googleMap;

    // Add a marker in Sydney and move the camera
    LatLng sydney = new LatLng(-34, 151);
    mMap.addMarker(new MarkerOptions().position(sydney).title("Marker in Sydney"));
    mMap.moveCamera(CameraUpdateFactory.newLatLng(sydney));
}

}

A:

As @tyczj pointed out, there's no v3 for Android Google Maps API, so you are probably using v2.
That said, to accomplish what you want, call setOnMapLongClickListener in your mMap object, and add the marker as you want inside onMapLongClick. You should do this in the onMapReady method:
@Override
public void onMapReady(GoogleMap googleMap) {

    ... 

    mMap.setOnMapLongClickListener(new GoogleMap.OnMapLongClickListener() {
        @Override
        public void onMapLongClick(LatLng latLng) {
            googleMap.addMarker(new MarkerOptions()
                    .position(latLng)
                    .title("Your marker title")
                    .snippet("Your marker snippet"));
        }
    }); 
}

// EDIT:
If you want to keep only one marker present at a time, you should declare your marker in the global activity's scope, and then, in onMapLongClick, if marker already exists, instead of creating a new marker, just update it's position:
public class advertiserMap extends FragmentActivity implements OnMapReadyCallback {

    // Declare marker globally
    Marker myMarker;

    ...

    @Override
    public void onMapReady(GoogleMap googleMap) {

        ... 

        mMap.setOnMapLongClickListener(new GoogleMap.OnMapLongClickListener() {
            @Override
            public void onMapLongClick(LatLng latLng) {

                // First check if myMarker is null
                if (myMarker == null) {

                    // Marker was not set yet. Add marker:
                    myMarker = googleMap.addMarker(new MarkerOptions()
                        .position(latLng)
                        .title("Your marker title")
                        .snippet("Your marker snippet"));

                } else {

                    // Marker already exists, just update it's position
                    myMarker.setPosition(latLng);

                }
            }
        }); 
    }
}

If done like this, remember to always check if your marker is not NULL before manipulating it in your code.

