Oracle Forms

I am trying use the ON-LOGON trigger to connect to the database
when a new form is called(from html page) by using the "logon(un,pw||'@'||db);" procedure:

I could use the 'userid=' parameter in thw URL to connect to the DB,
but I am trying to achieve this using ON-LOGON if possible.

I have declared a parameter in the form that will hold the connection string
value passed to the form through the URL. I can see this value if I call it from
WHEN-NEW-FORM-INSTANCE but I get the ORA-03114 if I use this value in the
ON-LOGON trigger to login.

Is there a way to pass the connection string to the form at runtime through the
calling URL?

Any suggestions?