Thank you for introducing me to the MutationObserver, but my issue still persists.
I managed to find the object, but when I insert the JS function 'onload' in the Basic Form, it is still not being called because the page loads before the subgrid within the same Basic Form. So far, I have only managed to implement the change by using a timeout. However, this solution is a bit crude, as it will display the dropdown for a split second instead of the button.