You need to use an HTML <textarea>
element.
From MDN:
The HTML
<textarea>
element represents a multi-line plain-text
editing control.
Using <input>
for multiline text is not possible natively and, if hacked, would be invalid HTML.
HTML5 spec:
4.10.5.1.2 Text (
type=text
) state and Search state
(type=search
)The
input
element represents a one line plain text edit
control for the element’s value.(emphasis mine)
Twitter input box
You mention you want the textarea to resemble Twitter’s (auto-resize / no scrollbar). Consider this option and the following SO posts:
A small, stand-alone script to automatically adjust textarea height.