As I'm working intensive with external ressources which's content is not (yet) being able to shown within BS Preview I would suggest to implement an "Auto-Export-On-Changes" Function, so it's possible to reload the complete page in external browser.
Better way of course would be to redesign the handling of external ressources used in BS, so also the Preview can display the results of an fetch - action.
To provide the full qualified url (https://www.webserver.tld/...) will not be an solution as this will be blcoked by CORS Security Policy.
Best would be, also to link the external e.g. php-files as ressources into BS, that way they are exported also into the final structure and can be reached from BS-Preview-Webserver.
This feature has already been asked for quite some time ago by myself and a couple others. Basically what we asked for is the ability to link local sources to our projects. I am pretty sure everyone that makes a website (well that makes one manually, not in an online builder) has a directory on their computer to hold the files that are being uploaded. I asked to be able to link a directory to the project so that BSS would look for resources within those directories as well as the ones within BSS system. Haven't seen anything posted on this in a while, but ... this would solve a lot of issues with JS, PHP, etc. without having to incorporate them into BSS at all as the Browser preview could then be set up to utilize the files without importing them into BSS and the PHP and other languages could be used as Custom Code block types if needed.