get Sign Up Payload

@CustomParametersOrder(order = ["currency"]) // optional
override fun getSignUpPayload(
     serverSecret: String,
     name: String,
     surname: String,
     email: String,
     password: String,
     language: String,
     vararg custom: Any
): Params {
     val payload = super.getSignUpPayload(serverSecret, name, surname, email, password, language)
     payload.addParam("currency", custom[0])
     return payload
}