@ajitabh_srivas are you able to create lookups as dropdowns based on the other list?

Why is the other list in a completely separate site?

What are you really wanting to do for the user on this form?

No i cant create lookups as they are in seperate site collection.

They are in seperate site collection related to HR data where as i want to access the same from apps site.

I am creating forms to be filled by emploees and want to use these lists from seperate site collection to autopopulate user data.

When i try to use JSOM it gives 403 error as token context is not created for destination ste collection. 

Any help??

Due to the apparent sensitivity of data and separation... it may be easier to just replicate the list into the apps site and then do a batch job to keep the list in sync and build as normal. 

This would eliminate the need for javascript altogether and let you use the forms product as intended.

We want to avoid replicating the data as we want single source of truth. Keeping data in different location in sync is also a task. I can implement this is html javascript as well, is there any way in nintex forms?