You can find the information on how FreeBets work in article 9.3.16 of the Terms and Conditions ("General FreeBet Conditions"). It states the following:
🎁 A Freebet is a free bet that allows players to get a return in bonus balance (subject to the Bonus Playthrough);
🎯 The Freebet is used in a single bet, either a single or multiple bet.
⏳ The Freebet has a validity of 7 days. If not used within this period, the offer will be removed.
🔄 If a bet made with a Freebet is canceled, the value will be refunded in the form of an equivalent Freebet.
🚫 The Freebet cannot be used in combination with any other promotional offer.
💡 Example: If you use a €10 Freebet on a bet with odds of 2.50 and the bet wins, the return in bonus balance will be €25. To convert the winnings from the Freebet into real balance, you need to complete the Playthrough.
Also, see the article: "How to use the Freebet I won?"