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$field         = \ProcessWire\wire('fields')->get('yourFieldName');
$fieldtype     = $field->type;
$options       = $fieldtype->getOptions($field);

$option        = new \ProcessWire\SelectableOption();

$option->title = 'Your select option title goes here';

$options->add($option);

$fieldtype->setOptions($field, $options);
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Tim Bennett is a web designer and developer. He has a First Class Honours degree in Computing from Leeds Metropolitan University and currently runs his own one-man web design company, Texelate.