Custom field, set to required

I would like to make people choose between left and right using a single selection list, and I need them to force them to make a selection. See screen print for how I aim to achieve this, offering them 3 choices, of which the default option is disabled.
However, the default option is accepted when placing an order. How can I achieve this?

I know that I can force users with a radio-button list, but I need this to work for single selection lists, as there are more fields I want to add, with more options.

This is testing on a localhost, so I cannot supply a link.

You should have the Waarde field of your Kies value empty in your custom field.

That's what I did earlier, but even when leaving the field blank, it automatically chooses the first value as the default one, see an attachment of a sample of what happens.

You completely removed the Kies value from the dropdown. That's not what I said.
I said that you should still have it, and still selected by default, but with an empty Waarde value.

OK, now I see, thanks!