Obtain the file iframeResizer.min.js and copy it to your web server.
Make sure to use the same version number as the iframeResizer.contentWindow.min.js which has been included on the booking page.(in this example 3.5.3)
The file can be downloaded from the developer site or from https://beds24.com/include/iframeresizer/3.5.3/iframeResizer.min.js