How to change the default permalink structure for new WPMU sites?

Hello & good day to you :slight_smile:

I have just installed a WPMU today and started to add a new site. I realized that this new site is using a different permalink-structure than the structure of my main site:
/%year%/%monthnum%/%day%/%postname%/

I already had changed the permalink-structure for my main-site, but I'd like all new sites to have
/blog/%postname%/
by default.

How can I achieve this?

Thank you and kind regards,
Sascha