i want to create a plugin for freichatx. But i don't know how to create a plugin for freichatx.
I want handwriting message plugin just like in cometchat.
Please tell me the procedure.

it is up to you how you create a plugin.
just add a button which pops up a dialog.you can customize this in freichat.js, don't forget to look at how other plugins work.
you might want to place all your plugin files in freichat/client/plugins
But the most important part would be coding and that is completely up to you how you implement it.