VBA Web Form Scraping - Blank field

Hi everyone,

I'm using VBA to populate a web form and 99% of it is working just fine.
I have one problem, though, when trying to populate a field that depends on a previous one. This is an internal (corporate) web form so I can't use a real example, but here is an exercise:

Imagine I want to populate two fields: one being the "Month" and the other being the "day". No matter the data I enter in the Month and although returning no error, the day field is always left blank.
I'm afraid this is somehow related to some kind of validation (as you know, a standard use of the form would't allow me to select February and then have have the day 31st available...). I even tried thing like sending a tabkey between both, just in case, but it still wont work.

Here's the code I'm using (fictional web site address):

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Anyone have an idea about this?
Thank you in advance!