Within Altium Designer, at the upper right hand corner.
Fig. 1 - Open workspace within Altium Designer
Connecting From Altium Designer
Start by logging into your Altium Live account in Altium Designer. You can check your login status in the upper-right corner of the Altium Designer interface. You should see your avatar. If you don't see it, click Not Signed In, select Sign In, and enter your Altium Live account credentials.
Fig. 2 - Interface when logged in and not logged in to Altium Live account
Fig. 3 - The Sign In option
Once logged in, you can freely use the Altium 365 Workspace features. Let's log in to a workspace.
After logging in to Altium Live, click Not Connected to access your workspace options.
Fig. 4 - Workspace options
Click on the desired workspace to connect to it. Once you have connected, the workspace name will appear in the upper-right corner, and you can freely use all its functionalities.
Fig. 5 - Connection to a workspace is complete, and the workspace name is displayed
Tip: In your Altium 365 Workspaces settings, found at https://profile.altium.com/workspaces/, you can set a default workspace. Altium Designer will automatically connect to your default workspace when it starts.