first you're gonna go to this link to get free proxy api:
https://docs.proxyscrape.com/


copy the link below " get proxies list "
it should look like this :
https://api.proxyscrape.com/v2/?request=displayproxies&protocol=http&timeout=10000&country=all&ssl=all&anonymity=all

then go to the target app 

for example:  in silverbullet/openbullet you have to go to proxies , click import ,  then click API and put the url you got from  the proxyscrape website.

then go to settings>proxies> put the reload interval in minutes to 5.