Open applications/donate/sources/Reward/Reward.php
Find:
$form->add( new \IPS\Helpers\Form\NumberRange( 'reward_amount_range', $this->rid ? array( 'start' => $this->amount_range1, 'end' => $this->amount_range2 ) : array( 'start' => 0, 'end' => 100 ), FALSE, array( 'start' => array( 'decimals' => 2 ), 'end' => array( 'decimals' => 2 ) ) ) );
Replace With:
$form->add( new \IPS\Helpers\Form\Custom( 'reward_amount_range', $this->rid ? array( 'start' => $this->amount_range1, 'end' => $this->amount_range2 ) : array( 'start' => 0, 'end' => 100 ), FALSE, array( 'getHtml' => function( $element )
{
return "<input type='text' value='{$element->value['start']}' name='reward_amount_range[start]' min='0' class='ipsField_short'> - <input type='text' value='{$element->value['end']}' name='reward_amount_range[end]' min='0' class='ipsField_short'>";
} ), NULL, NULL, NULL, 'reward_amount_range' ) );
Save.