querySelector or querySelectorAll with an attribute selector should do it:
// The first element that matches (or null if none do):
var element = document.querySelector('[aria-label="Message Body"]');
// A list of matching elements (empty if none do):
var list = document.querySelectorAll('[aria-label="Message Body"]');
Or if that ID is stable, simply:
var element = document.getElementById(":ik");
(Note that the d is lower case; you have it in upper case in your example.)
Either way, make sure your code doesn’t run until the page is loaded, by including this in your manifest:
"run_at": "document_end"
(A little) more in this answer, which references this Google documentation.