You can install TicketBook to your Jira Cloud instance in two ways: Install the app from Atlassian Marketplace or Install the app from Jira admin pages.
Install the app from Jira admin pages:
Open Settings >> Jira Settings >> Apps >> Find new Apps
Enter TicketBook in the textbox. Press Enter.
TicketBook will appear in the search results. Click on it.
Click on Get App.
"Add to Jira" window will pop-up. Click on Get it now.
Jira will install the app and the free app license.