how to add our company GST number

If we pay for the entire year for an additional year for iCloud extra space

Is not possible.

If you have an iCloud subscription, you will receive a monthly bill from Apple, check your email box.

You pay monthly for iCloud storage.

how to we claim GST for a refund. Should we provide our GST numer?

Every company should know how to do that.

Call Apple in your region see: Contact Apple for support and service

Contact Apple for support and service - Apple Support

Contact Apple Support -> Get Support (apple.com)