I want to make fractional box

as the picture, how to drawing only the first line of the paragraph for

var box1 = app.selection [0].texts[0].insertionPoints.item(-1).textFrames.add();

box1.geometricBounds = [0, 0, 6.4, 7];

var y1 = "1234"

var y2 = "\r5678";

var txt = box1.texts[0].insertionPoints.item(-1).contents = y1 + y2;

box1.texts[0].underline=true;  //Paragraph ???

box1.underlineOffset = 1; // Paragraph ???

box1.anchoredObjectSettings.anchorYoffset = -2.283;

box1.texts[0].appliedFont = "Swis721 BT\tRoman";

box1.texts[0].pointSize = 8;

box1.texts[0].justification = Justification.CENTER_ALIGN;

box1.texts[0].leading = 12;