import json
value = 'J590'
# Using `print` in place of `driver.execute_script` just for this example
print(f"document.getElementById('CustCd').value = {json.dumps(value)};")
aW1wb3J0IGpzb24KdmFsdWUgPSAnSjU5MCcKIyBVc2luZyBgcHJpbnRgIGluIHBsYWNlIG9mIGBkcml2ZXIuZXhlY3V0ZV9zY3JpcHRgIGp1c3QgZm9yIHRoaXMgZXhhbXBsZQpwcmludChmImRvY3VtZW50LmdldEVsZW1lbnRCeUlkKCdDdXN0Q2QnKS52YWx1ZSA9IHtqc29uLmR1bXBzKHZhbHVlKX07Iik=