I assume you're talking about the pre-installed Samsung Messages app, and if so, here's what you need to do:
Open the app drawer (usually the icon on the far-right side of the dock on your homescreen, and it looks like a grid of dots on a white background)
Scroll through the list of apps until you find the text app you normally use
Press and hold the icon, then drag it to the homescreen of your choice (or the dock if you'd prefer)
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