There’s not really a standard way to format text messages in your script. But we understand you want it to flow naturally and look well presented.
Most of the ways to deal with the issue of how to format text messages in your script are actually correct as long as they are clear and shows the intention of the writer, not leaving the reader lost and confused.
One easy way to combat this is to treat the text messages as dialogue and format as shown below:
Overall the director usually chooses how the text messages are portrayed. This may be a voiceover or an on-screen display. Whichever way the director chooses it will sit aligned with the dynamic of the program. However, you can hint to the way you wish to portray it.