How does one send a chat message in Facebook using jquery within the Chrome browser?

How does one send a chat message in Facebook using jquery within the
Chrome browser?

I'm trying to send a chat message in Facebook using jquery, but keep
getting an error message. Either it being "TypeError: Object # has no
method 'WHATEVERIPUTHERE'"
[Note that I'm pretty new with coding.]
// Basically o = the chat text area.
// the o.WHATEVERIPUTHERE("Hello!") is what I'm trying to do (setting a
value for the text area, and sending it)
I have no idea how to get this to work as my method knowledge is limited.
Basically, I just need help exapnding on this to make this a workable
code. Thanks! [code below]
var o = document.getElementsByClassName("uiTextareaAutogrow _552m");
o.WHATEVERIPUTHERE("Hello!");