I accidentally figured out what the gesture is you use when using voiceover to dismiss the edit box in iMessage. I have not tried this in other apps yet, but here is the gesture. It’s sort of complex, but it works.
Make sure your finger is sitting just above the edit box. You have to drag your finger around just above the edit box if you have already been in the edit box. When you find an area that sounds like nothing when using voiceover, then you Double tap, hold, and slide your finger down. You slide it down until you hear the sound that voiceover makes when something is dismissed. This is useful if you want to find the send button easier. The send button should be located on the bottom right hand side of your screen if there’s text in the box. If there is no text, there will be the record audio button instead if you are using iMessage. This gesture also works for text message. Hope this helps.