After doing that, switch to another website or application and paste emoji using context menu or keyboard. Select this like a regular text, and copy to the clipboard. All you need is to select, copy and paste this symbol. It's very easy to get Rainbow Flag Emoji both on computer and mobile without any emoji keyboard installed. You may use this Copy and Paste Emoji List to copy/type emoji one-by-one or this Online Emoji Keyboard to write all your message at one time and copy it when finished. Easiest way is to copy & paste, but other methods, like windows alt-codes and HTML-entities (for web sites) are also popular enough.
There are number of methods to type Rainbow Flag Emoji.
