in auth-lib/src/main/java/com/spotify/sdk/android/auth/AuthorizationRequest.java [110:119]
public Builder setCustomParam(String key, String value) {
if (key == null || key.isEmpty()) {
throw new IllegalArgumentException("Custom parameter key can't be null or empty");
}
if (value == null || value.isEmpty()) {
throw new IllegalArgumentException("Custom parameter value can't be null or empty");
}
mCustomParams.put(key, value);
return this;
}