Sending changed constant values in Simulink external mode

I am trying to program a "master" STM32 board using Simulink to send a uint16 value via UART/Serial Communication to a "slave" STM32 board, which is programmed with Arduino IDE. The Rx of both boards are connected to the Tx of both boards. Both boards are connected via USB to the same PC. My current Simulink model is as follows:

I have a problem decoding it, because STM32's SCI write block doesn't have an option to add a trailing line feed to indicate the start/end marker of the bytes being sent. I am wondering, if it is possible to send the data if the constant values in Simulink (external mode) has been changed? If so, how? Thanks.