Thanks for writing to us. As of now, there is no seprate option to create collection while saving JSON document, will taking your request and add this feature in our dashboard to create collection. Right now, you can save multiple collection from dashboard by writing name of your collection in save json form, enter the required field and click on submit button to save your new collection in App42 data base.
Let me know if it helps.
P.S Once this feature is avaiable in dashbaord, will update this thread.