Both can be valid depending on intention. If you've already discussed the idea of adding webinar functionality, and there is a specific kind of functionality decided, and it's now just a case deciding to add or not to add, then you can refer to it as "the webinar functionality" because its clear "which" webinar functionality is being referred to. But if you've not yet discussed it, then you would not use "the", because "the" implies there is a specific one already on the table. If you use "the" wrongly, people may ask "which webinar functionality exactly?" This is why "the" is called "the definite article", linguistically. Because what it refers to, should already be clear/defined.