Update form submission field value on redirected Thank you/Welcome Eloqua page

Summary:

Hi,
I am trying to populate first name value from the form submission on the redirected Welcome Page. My set up looks like: I have a First Name field in the form with first name field merge mapped as prepopulate field data. After form submission, the redirected landing page has a dynamic content with a message as "Welcome First Name field merge". When tested within the network, it was populating the value for first name however, when tried submitting outside the network, it shows no value for First Name on the Welcome Page. Also, tried first name field merge directly on