Либо
$("#threshold").append($("<option>",{ value: "90",text: "90%", selected:true }));
$("#threshold")
.append($("<option>",{value: "70",text: "70%"}))
.append($("<option>",{value: "80",text: "80%"}))
.append($("<option>",{value: "90",text: "90%", selected:true }))
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<select id="threshold">
</select>
или
$("#threshold")
.append($("<option>",{value: "90",text: "90%"}))
.val("90");
$("#threshold")
.append($("<option>",{value: "70",text: "70%"}))
.append($("<option>",{value: "80",text: "80%"}))
.append($("<option>",{value: "90",text: "90%"}))
.val(90);
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<select id="threshold">
</select>
короче:
const curTH = 90;
$.each([70, 80, 90], (_, item) =>
$("<option>",{ value: item, text: item + "%", "selected": item === curTH ? true : false })
.appendTo("#threshold")
)
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<select id="threshold">
</select>