How to activate a Warframe Manual Delivery key?

Activating a Warframe Manual Delivery key is really simple, just follow these steps:

If you were playing the game log off Warframe, and also log off your Steam account.
Log in to Steam with the credentials provided in an email you received after purchase or in your library on our store.
Log into your Warframe account (using your own login and password) from the Steam account of the seller.
You will see information: You have pending Steam packs. Would you like to apply them to this account? Click: Accept
If you're requested to Link to Steam economy - Decline
Once you accept the Steam pack the content will be added to your Warframe account.
After this relog to your Steam account and double-check the added content.

DLC is not visible in Steam library, it will be visible in-game in Warframe.

This code can be used only on a new Warframe account that has not been linked with a Steam account.

The key is not working

If you have activated the key according to the guide but the code appears not to work, please check what type of error message you receive and depending on it follow the appropriate instructions: