forbidden error 403 when i use an external url in my site!!

Question

hi

i have written a website for download free softwares

i put an ability to use external urls from other websites to my website as indirect url to download the softwares.

but when i click on download button in my website, it has 403 forbidden error! 

how can i fix it? i know it's because of accessibility of that website, but is there any solution to solve it? 

if i paste the url in internet download manager or the other downloader softwares, it done successfully but from my download button not!

please give me the best solution

thanks a lot