In After Effects, go to File → New → MAXON CINEMA 4D File...

Choose a filename and save location. Click Save.

In the Cinema 4D License dialog, click OK.

In the License Manager dialog, go to Sign up here.

Create your MAXON account, confirm your email, and sign back in to the License Manager.

In the License Manager, next to Cinema 4D - Lite, click Activate.

<aside> ⚠️ If you don't open Cinema 4D using After Effects as described above, you won't see Lite as an option here.

</aside>

Here's what a successful activation looks like! To open Cinema 4D Lite from here, click OK and Cinema 4D Lite will open.