smart.js

async function addSubTextPart() {
    var range = await getSelectionRange();

    var name = document.getElementById("subTextPartName").value;
    TXTextControl.subTextParts.add(name, 0, range.start, range.length);
    TXTextControl.focus();
}

function getSelectionRange() {
    return new Promise(resolve => {
        TXTextControl.selection.getStart(function (curSelStart) {
            TXTextControl.selection.getLength(function (curSelLength) {

                var range = {
                    start: curSelStart + 1,
                    length: curSelLength,
                };

                resolve(range);
            });
        });
    });
}