[BulliM]

I use some JS for. You can use something like this, to select an other option by default:

if ($("h1:contains('Create')")) {
    $('.js-content-entry-prefixes option[value="ASK:"]').prop('selected', true);
    $('.selectBox-label').first().html('ASK:');
}

Not tested, if it's worked with colons! May to escape them.

If it is needed to delete 'No prefix', you can use CSS:

ul.selectBox-dropdown-menu li:nth-child(1) {display:none}