After adding dedicated ip on server, go to Plesk admin -> Tools & Settings -> IP Addresses -> Reread IP to update list of IPs.

On list of IPs, select Resellers of specific IP you want to assign to domain (it should show 0 for unassigned dedicated IP), then click Assign and select reseller of domain.

Search reseller account owning domain, go to IP addresses link under More  Tools to see new dedicated ip assigned to reseller.

Search domain you want to assign dedicated IP, go to Subscription settings -> "Change Hosting Settings" -> select dedicated ip in drop down and click OK to update IP for website.

Plesk will update website binding in IIS and domain dns records to dedicated IP.