Dropdowns do not update when values are set in backend

[Webberjo]

If I set my country dropdown's value to "Canada" in the backend, the dropdown shows the selected country as "Canada", but the region dropdown still displays "-" as if a country hasn't been selected.

Update
To get the region dropdown to populate, I had to do this:

document.getElementById('country').dispatchEvent(new Event('change'));

[papagunit]

Thank you! This is exactly what I needed. I have a separate prepopulation script that puts a value in the country field, but it doesn't trigger the region specific list, so the region stays uninitialized, and results in not it being populated with the other given value.