An alt is a second Roblox account. Adding it means its playtime, likes and trade ads all count for one RoRetro account.

1. Link your main first (see Link your Roblox account).
2. Open the RoRetro popup and press Add another Roblox account (alt).
3. In Roblox, switch to your alt (log out and into the alt, or use Roblox's account switcher).
4. Press Join the RoRetro game and type the new code there, on the alt.

Picked "An alt account" before linking a main? That's fine: RoRetro links your main first, then gives you a second code for the alt.