To use the €5 Bet Builder Free Bet, your bet slip must contain at least one Bet Builder selection. This means you need to combine at least two markets from the same match (e.g., "Home Team to Win" and "Over 2.5 Goals").
Once you have created a valid Bet Builder bet, you can select the €5 Free Bet in your bet slip and then enter your stake in the corresponding field.
You can find more information about Bet Builder bets here.