Aubiree's post is probably relevant to persons who eat in the restaurant and want to take the dessert with them - not to those who just want to walk up to the counter and order a dessert to go. I am pretty sure you won't be able to do that at all. I'd try having the dessert served, eat a bite, then ask if you can take the rest with you.
There've been reports on Olivia's that they allow no takeout orders at all. You can order anything from their menu in the Gurgling Suitcase bar if you want to - however, you're required to eat at least some of it in the bar, you can't get it strictly for takeout.
Disney TS restaurants are not equipped to accommodate full takeout orders and probably will not do so. You may have varying levels of success if you are already seated in the restaurant. You will probably be able to take away a part of the meal you have already been served, but asking for extra items "to go" may not be permitted.