You should have a dictate button on the bottom slightly to the left of the space bar when the keyboard is shown. You can also ask Siri to do it, too.

Apart from engaging Dictation from the keyboard, with VoiceOver turned on, any time you are focused on a text field, you can do a magic tap/two-finger double tap and that will start dictation. Another two-finger double tap will stop dictation and you'll hear the feedback for what you dictated. Super handy.

There's a dictate button on the keyboard which is usually one to the right of what voiceover says is the more symbols button, which switches between alphabet to symbols and numbers. Press that, say what you have to say, then double tap to stop, or two finger double tap if you lost focus on the stop button. Here's a protip about dictation on iPhone. Don't talk into the button mic, it's the best sounding one on the thing, but it isn't the one it uses. It uses the one in the ear piece, so hold your phone sort of out from you just a bit and down a bit so you're sort of talking into the top part, and the accuracy will be better.

For my iPhone, I can activate Siri and say, "Message (recipient) (Message i.e. 'are we meeting tonight')". This works on my iPad Pro too, but as a stutterer, I don't make much use of this feature.