You need to be a Confluence Administrator in order to install Embedder.
First, login as the Administrator to your Confluence instance and select Apps from under Confluence Administration by clicking on ⚙️ icon at the top right corner.
In the Find new apps page, search for Embedder for Confluence.
Click on the Try it free button on the right side of the corresponding listing
This will result in the Embedder for Confluence app being installed and activated.
A confirmation dialog will tell you that the installation was successful.