Function create_user_defined_message 

pub async fn create_user_defined_message(
    configuration: &Configuration,
    account_sid: &str,
    call_sid: &str,
    content: &str,
    idempotency_key: Option<&str>,
) -> Result<ApiPeriodV2010PeriodAccountPeriodCallPeriodUserDefinedMessage, Error<CreateUserDefinedMessageError>>

Create a new User Defined Message for the given Call SID.