Request default interest for clients as a lawyer

As salaamu alaykum dear brothers and sisters,

I am a lawyer in Germany. Here it is common for a debtor to have to pay default interest to the creditor if he ist late with a payment (not loan, any payment). The amount is based on the ECB prime rate. If a case goes to court, default interest usually is claimed in addition to the principal claim. As a lawyer, am I allowed to sue for this on behalf of my client?

Jazakallahu kheiran