How to use Pass URL in the Downloads

I wanted to send download request to a CPE using below Provision script.
I wanted to pass the URL , not use the default one, is there a way to define the URL , if so please let me know what is the right way to define it??

declare(“Downloads.[FileType:1 Firmware Upgrade Image]”, {path: 1}, {path: 1});
declare(“Downloads.[FileType:1 Firmware Upgrade Image].FileName”, {value: 1}, {value: “firmware-2017.01.tar”});
declare(“Downloads.[FileType:1 Firmware Upgrade Image].Download”, {value: 1}, {value:});

I have not found a way to set a own download URL.
I have the situation where I should use different file service for different CPE’s, so I had a look into this topic. (So I was not able to solve that and found a more or less working way for my issue)

It is possible to change the base part of the URL for the genieacs FS service with an env variable. This is what I used to change the base URL to an other server for all devices.

Could you pleas share that env variable ?