Yes, it is possible to trade on margin from DEGIRO. This service is referred to as Debit Money.
Trading on margin involves investing more than the cash value held in your MMF. When trading on margin from DEGIRO, your MMF will be negative. With the Active profile, a more conservative approach to calculating portfolio risk is taken. This means you will have less available margin.
With the Trader profile, a less conservative approach to portfolio risk is taken. The Trader profile allows you to trade with more margin from DEGIRO, however this carries greater risk.
For full information please see the document Debit Money Conditions.