Unfortunately, it is not possible to directly highlight or modify the appearance of the send button within the standard MFMessageComposeViewController. This is because the interface provided by the MFMessageComposeViewController is not directly customizable and is intended to provide a consistent user experience across all iOS apps.
One solution to consider is to provide additional instructions or guidance to the user before they are presented with the MFMessageComposeViewController. This could include an on-screen message or tutorial that specifically points out the location of the send button within the interface, or providing a visual indicator such as an arrow or highlight box to guide the user to the correct location.
Another option is to present the MFMessageComposeViewController in a modal view controller and add additional instructions or guidance in the modal view controller.
Also, you can use the accessibility options to add a VoiceOver label to the button, which can be read out loud to the user when the button is in focus.
It’s worth noting that Apple’s HIG (Human Interface Guidelines) discourages from customizing the system’s standard view controller. You should try to stick to the standard iOS interface so that the user can feel comfortable and familiar with the app.