silverstripe-userforms/code/modifiers/UnderscoreSegmentFieldModifier.php

<?php

use SilverStripe\Forms\SegmentFieldModifier\SlugSegmentFieldModifier;

class UnderscoreSegmentFieldModifier extends SlugSegmentFieldModifier {
	/**
	 * @inheritdoc
	 *
	 * @param string $value
	 *
	 * @return string
	 */
	public function getPreview($value) {
		return str_replace('-', '_', parent::getPreview($value));
	}

	/**
	 * @inheritdoc
	 *
	 * @param string $value
	 *
	 * @return string
	 */
	public function getSuggestion($value) {
		return str_replace('-', '_', parent::getSuggestion($value));
	}
}