Tap the icon (often found in the "Store" menu or by tapping your profile/account balance). Select "Redeem a Steam Gift Card or Wallet Code" . Enter your game key into the text box and tap Continue .
If you can’t find the menu options, you can use the app’s built-in browser: steam mobile activate key
If you find the app's menus confusing, you can use the official Steam Product Activation web page in your mobile browser. [TUTORIAL] How to activate SteamKeys through the steam app Tap the icon (often found in the "Store"
: Select the option "Redeem a Steam Gift Card or Wallet Code" . steam mobile activate key