$fieldset->addField('field_name', 'text', [ 'label' => 'Field Label', 'name' => 'field_name', 'required' => true, 'value' => 'Default Value' ]);
$fieldset->addField('field_name', 'select', [ 'label' => 'Field Label', 'name' => 'field_name', 'required' => true, 'values' => [ ['value' => 0, 'label' => 'Option 1'], ['value' => 1, 'label' => 'Option 2'], ['value' => 2, 'label' => 'Option 3'] ] ]);This code creates a select box field with a label, name, required validation, and three options to select from. Package library: Magento Framework (Magento_Catalog module)