When editing the Send Email action in the Send Email Editor dialog, the Insert Field button ignores the cursor position and always inserts the selected field token at the end of the text.
To solve the issue:
editor.set_html(Sitecore.EmailEditor.__RAD.scGetValue() + "[<label id='" + element.id + "'>" + element.innerHTML + "</label>]");
editor.pasteHtml("[<label id='" + element.id + "'>" + element.innerHTML + "</label>]");