Использовать атрибут defaultValue
,
<Select defaultValue="MT"/>
Пример параметров веса:
const weightOptions = ["MT", "KG"];
const addWeightOptions = weightOptions =>
weightOptions.map((weight, i) => (
<Select.Option key={`${weight}-${i}`} value={weight}>
{weight}
</Select.Option>
));
<Select
placeholder="Select Weight Units"
defaultValue={weightOptions[0]}
>
{addWeightOptions(weightOptions)}
</Select>