How to get variable form data from a field created by Javascript ?

Hello,
During first access to our application, the first page is loading a form (with all fields hidden) and a javascript (JS) which run a function to generate a kind of crypted variable PATHPARAM.
The JS create a new field into the form with the new crypted value PATHPARAM, then submit data.
At Neoload, we get the value of PATHPARAM but it's static and we didn't succeed on make it dynamic, which cause the fail to access into the application : we can't run any recorded scenario.
Any help on this issue ?
Let me know if you need any further information. Thanks