You can go a maximum of 33% below your squad value plus the amount currently in your account.
Example:
Suppose your squad value is €100 million and your account is €10 million in the negative. Your account balance cannot fall below -€30 million. This corresponds to 33% of a squad value of €90 million (€100 million minus €10 million).
Calculation:
€100 million squad value and -€10 million account balance
Squad value + negative balance = €100 million + (-€10 million) = €90 million
-> 33% of €90 million = €30 million
Therefore, your account balance cannot fall below -€30 million.
If you cannot overdraft your account further, you will receive a notification in the app when submitting a bid that your squad value is already 33% in the negative.