Add a filter to lookup field in form page

Hello Community, 

How can we add a filter to a lookup field in a form page so it can show only the required fields? I have tried by updating it in the client schema by using the code and it worked fine but whenever I add or modify any other field in form page, the code in client schema disappears. I have also tried using business rule, but we don't have an option to add such filter. 

Any suggestions are really helpful.

Thanks

Gargeyi.G