It seems that in the Enterprise addition, when I create a mass mailing, I should be able to add a custom field. I am trying to do this by selecting the studio option in the top left, and adding a name field, but I cannot see the body of the message I am working on. Instead, it shows the error message that the URI is too large. 

I tried this even in the basic template without changing anything, and I still see the same message. Do I have to remove all links before doing this? 

Before trying this, I looked at other's answers for Dynamic Fields, and tried the code mentioned, but it does not work when pulling from mailing lists instead of contacts. 

Thank you in advance