rect = new fabric.Rect({
    left: 0, top: 0,
    width: 300, height: 75,
    fill: '#D5FDE6', stroke: '#2ECC71'
});

t1 = new fabric.Textbox('Signature*', {
    top: 40,
    left: 10,
    fontSize: 16,
    fixedWidth: 150
})
viewer.pages[1].fcanvas.add(new fabric.Group([rect, t1]));