Paypal Open in New Window

I would like paypal to open in a new window -blank, so that the client site does not close. How can I do this?
Thanks

In order to do that change without any hack of HikaShop you will have to do that:
Copy the file plugins/hikashoppayment/paypal_end.php to templates/YOUR_TEMPLATE/hikashoppayment/paypal_end.php via FTP.
Edit the file templates/YOUR_TEMPLATE/hikashoppayment/paypal_end.php and change the line:
<form id="hikashop_paypal_form" name="hikashop_paypal_form" action="<?php echo $method->payment_params->url;?>" method="post">

to:
<form id="hikashop_paypal_form" name="hikashop_paypal_form" action="<?php echo $method->payment_params->url;?>" method="post" target="_blank">

I would like to do the same and i've tried to do the changes you're suggesting, BUT at tempelates/mytemplate there is no hikashoppayment. Do i have to create it? Sorry for being newbie

Yes, you have to create the folder.