On my s7 I used to be able to be able to use speech to text by pressing a small mic symbol. On my S9 I can no longer find that symbol anywhere. There is no settings symbol next to the space bar on the keyboard when it's open.
When you launch your stock Samsung Text messages app and press on the text space to invoke the keyboard at the top of the keyboard you should see a symbol with two arrows circling it.
Tapping on this changes the top bar of the keyboard to show different options with the Mic symbol being one. Tapping on it again shows any predictive text.
If you can't see it then press the cog icon and then Keyboard Layout and Feedback and toggle On Keyboard Toolbar.
Then launch your keyboard again and at the side of the cog is a 'v' option.
Press on that and it should give you options on how to order the icons you want in the keyboard toolbar.
I could not get the microphone to show on my keyboard anywhere but I did find an alternative solution to this problem using Bixby. You have to enter the Bixby settings and turn on the toggle for Bixby dictation. Then when your keyboard is open hold the Bixby key and you can dictate. Hope this helps!
You can also hold down the spacebar to bring up the Speech to Text mike. Tap the cogwheel at the top right on the keyboard screen, then select "Swipe, touch and feedback" and it should be under touch and hold space bar. If it's not a default, you can select it. Good luck.