I still cannot do too much here because this relates to a third party scripting and I am not familiar with this product or how it works. As for support, wherever or whover you are getting this shop from should be providing support, whether it’s from Plesk or your webhost. So wherever you set this up from, that is where you get support. If you don’t have support from anyone for this shop system, then you need to find something different that does offer support because running an ecommerce site should have it. If this shop is a service from your web host, you need to contact their support. You might also want to do Google search for what you need to do, or find out if Plesk has a support forum for this shop system.
By the way, if the submit or sign up button is part of the form for this functionality, your button needs to be within the