I'm trying to achieve the same in my project. I found the following in the official x-editable documentation:
Creating new record is a bit problem for editable interface because primary key (record id) is unknown and in-place-editing has no related object on backend. In that case editable will store new value inside and will not submit anything to server until you do it manually .This is default behavior when pk option is empty and send='auto'. To submit manually several editable fields there is submit api method.
Here is the link: x-editable documentation
It shows you an example also how to handle such case.