gistfile1.txt

// add dependency to gradle
implementation 'com.android.installreferrer:installreferrer:1.1'

// add variable inside Activity/Class 
InstallReferrerClient referrerClient;

// call this code
referrerClient = InstallReferrerClient.newBuilder(this).build();
    referrerClient.startConnection(new InstallReferrerStateListener() {
        @Override
        public void onInstallReferrerSetupFinished(int responseCode) {
            switch (responseCode) {
                case InstallReferrerClient.InstallReferrerResponse.OK:
                    // Connection established.
                    ReferrerDetails response = null;
                    try {
                        response = referrerClient.getInstallReferrer();
                    } catch (RemoteException e) {
                        e.printStackTrace();
                    }
                    // THIS IS WHAT WE NEED. Pass this string to our tracker
                    String referrerUrl = response.getInstallReferrer();
                    break;
                case InstallReferrerClient.InstallReferrerResponse.FEATURE_NOT_SUPPORTED:
                    // API not available on the current Play Store app.
                    break;
                case InstallReferrerClient.InstallReferrerResponse.SERVICE_UNAVAILABLE:
                    // Connection couldn't be established.
                    break;
            }
        }

        @Override
        public void onInstallReferrerServiceDisconnected() {
            // Try to restart the connection on the next request to
            // Google Play by calling the startConnection() method.
        }
    });