External link client variable parameter

Hello,

I built a web page to read some data from IFS (plsql calls with ODBC)
I need to add external links on IFS pages as Sales Part for exemple.
I need to pass two parameters into the URL.
One of those is the user language, available in client variable in events as ##USER_LANGUAGE##
My problem is I can not chose the parameter in the parameter list, it doesn’t show.

I also tried to set the parameter directly in URL using events syntax :
- lang=#USER_LANGUAGE##​​​​
- lang=${USER_LANGUAGE}
- lang=${#USER_LANGUAGE#}

I already had the same issue for the user id and I created a custom field which reads the user id throught PLSQL function, which is not a good way to achieve it on my opinion.

Do you know another way to achieve this ?

Thank you