When FamilySearch center guests' work in Family Tree, they can click a link to search a third-party website. If they see an option for the Institution version of the website, they can click it.
For other third-party subscription links, the website performs the search but prompts for a sign-in. The sign-in page happens because the link from Family Tree takes the guest to the regular version of the website. The guest must use the Premium Websites link in the FamilySearch Center Portal to search these websites.