Summarize the following:
Once you’ve selected the message you want to respond to, hit the button that says “Reply” at the top of the page. This will open a text box where you can write your message. When you’re satisfied with what you’ve written, click the yellow “Submit” button to send the message. You’ll need to limit your message to no more than 4,000 characters. If you’re sending the message from a web browser, you can also add an attachment, such as an image, a PDF file, or a Word document. Amazon won’t share your email address with the seller. All communications will be directed through Amazon’s messaging service.

Summary:
Hit the “Reply” button and write a message.