how to edit pluto sdr qpsk example

I am working with this example from MathWorks: https://www.mathworks.com/help/supportpkg/plutoradio/examples/qpsk-transmitter-with-adalm-pluto-radio-1.html

When i run the example it creates an sdrqpsktx variable in the matlab workspace

I want to change sdrqpsktx.MessageBits to something smaller. When i run the following code in matlab:

a = sdrqpsktx.MessageBits(1:448);

sdrqpsktx.MessageBits = a;

I successfully change sdrqpsktx.MessageBits to a. However when i run this in simulink sdrqpsktx.MessageBits changes back to its original size.

How do i permanently change sdrqpsktx.MessageBits and run the example with my changes?

Thank you.