Atlassian’s document describes how to install apps, including Slack Pinboard for Confluence:
Manage your apps (Confluence Cloud)
See Find and add new apps section for the details.
You can also add it from Atlassian Marketplace. Go to marketplace.atlassian.com and search Slack Pinboard for Confluence. You will find Try it free button at the upper-right corner.
Atlassian Marketplace: https://marketplace.atlassian.com/apps/1223210/slack-pinboard-for-confluence?hosting=cloud&tab=overview