So sorry for the delay, Hawaii kept me much busier than I thought! When it comes to text-like communication, things get a bit confusing. It seems like the HoloNet, the network used to send communications back and forth across the galaxy, could transmit messages between two people in two ways: more of a forum type of thing, or as something akin to an email. (Of course, you can also have a vocal conversation or talk to someone as a hologram, but these aren’t text-based.) The forums work pretty much like ours, so if you ever wanted a group chat sort of thing you could go with that. For text-based HoloNet communications between two individuals, it becomes unclear exactly how it works. We know of one HoloNet service called Imperial.emp in which people had their own contact addresses (Vader’s was rumoured to be vader@imperial.emp, which leads me to believe it was intended to function like email), so I can only assume that this is the closest thing to a texting system that the HoloNet has. I don’t know of a canonical format for either the forums or the individual messages. I also don’t know of any device that has been explicitly said to be able to send and receive text-based messages. Supposedly, anything with a keyboard and the necessary equipment to interface with the HoloNet could do it.
Outside the HoloNet, some planets/sectors/cities had their own communications networks in which people could transmit messages to other individuals. If both your characters are in the same general area, this would be the closest thing to a text message. However, by far the best way to get a message across the galaxy in any reasonable amount of time is by using the HoloNet, so if they’re a great distance apart you will want to stick to HoloNet services. Of course, you don’t have to stick to any of this canon stuff, especially since you will probably be making up your own format either way.
I hope that helps!
~ Jacen