How do you add a custom form field ( T-shirt size) and sync response to SFDC?

[Lea_Simpson]

I'm wanting to add a custom drop-down form field " Shirt Size" to a LP however, we currently do not have that field in our Marketo form field selection. How do I create the field without having to create a new field in Marketo that will syn to SFDC? 

[Michael_White]

If it absolutely needs to sync to SFDC, your best bet is to work with your SFDC admins to have it added there first, and enable visibility to your Marketo Sync User. 

But generally speaking, with something like T-Shirt size, it doesn't always make a lot of sense for that to permanently exist in your CRM (unless you're in the apparel business  ). If it's a scenario where you only need the information for a one-off event of some kind, you might want to consider other alternatives like creating a new Marketo-only field or just re-using an existing field in your instance and passing the information to your sales team (if they really need it) as a token in a task or alert of some kind.