Initially, you must install a mobile authenticator to protect your Steam account.
Mobile authenticator installation method is as follows.
1) Install Steam application for your mobile phone environment
Windows Phone Links
2) When the application installation is completed,
3) Select Steam Guard from the upper left menu of the application
4) Select the Add Authenticator button
5) Enter the number to receive the authentication key
6) Enter the authentication key sent to the entered number into the activated application screen
7) Check the recovery code
This recovery code is valuable information that is essential when you lost your Steam account or lost your Mobile Authenticator.
8) Once the setting is completed, each time you log in to Steam, an authentication key is sent to your registered mobile device.