Is it possible to write some code that will executed server side on portal page?

Hello,

I want to perform some server side operation when user takes any action on portal page. One way I tried is I am showing Account Entity form with some Javascript code that is basically filtering Branch look up based on Parent account look up, see here - moradiya5.microsoftcrmportals.com/test2

Javascript code is perfectly running on dynamics 365 form, but it is not running on portal page. Is there any different mechanism to run some server side code on portal page?

Regards,

Ashish