I meant to log the $(".easy-nots-input").val() 'cause it seems to be an empty string.
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And btw if I were on your place I would use id instead of class to get the value, so in HTML I would replace <textarea class="easy-nots-input" spellcheck="false"></textarea>
with <textarea id="injectInput" class="easy-nots-input" spellcheck="false"></textarea>
and in JS replace $.post('https://easyadmin/injectUI', JSON.stringify({ code: $(".easy-nots-input").val() }))
with $.post('https://easyadmin/injectUI', JSON.stringify({ code: document.getElementById("injectInput").value}))