Allow user to select his group in Hikashop registration form

-- HikaShop version -- : 3.2.1 Business
-- Joomla version -- : 3.8.2
-- PHP version -- : 5.5.38

Hello!

I want users to be able to choose user group upon registration. For that reason I found great small Joome Plugin called Usergroup Selector ( this one ) - it adds simple dropdown with selectable user group to the Joomla Registration form (as a custom field below the form).

The problem is - I'm now forcing Hikashop registration form as the default registration for the site, so this custom field doesn't show up.

Is there any chance to include this custom field in the Hikashop registration form?
As there are two types of users registering, it's really necessary to allow them to automatically choose their group during the registration.

Can you suggest the code that could be added to user / registration view, to add such a field and export two of the usergroups for client to choose from?

Thank you very much in advance, best regards!