The only way to use your friend's referral code is by clicking on the referral link they sent you or entering their referral code within 48 hours of creating your account. If you created an account without entering your friend's referral code you can still enter it within 48 hours by following the steps below:
- Tap on the three lines at the top left of the app
- Tap on Add Referral Code
- Enter your friend's referral code
- Click on Submit Referral Code
If it does not give you the option to enter a referral code it's because you already went through their referral link. Their referral link automatically ties you to them, so they will receive their referral bonus.