Want to install Windows with a USB stick on my DIY PC?

The following situation: I would like to install via USB stick Windows on my DIY PC, but my question is whether I enter the menu when you normally enter his license key me with my Microsoft account that I already use on my current laptop and no more license must acquire

You can say during the installation that you do not have a license key and then you have to choose your Windows 10 version (Home / Pro).

And you can log in later with your Microsoft account.

So do not I have to buy a license?

Depends on whether your license is linked to your MS account.
Unfortunately, I can't tell you how to do that.

Anyway, thanks anyway. I'll just buy a 15 Euro license from MMOGA

Can you do it, but most of the time you can't use the key during installation (must be enabled after installation)

Okay, thanks for the hint

Where do I activate the license code?

Either via CMD
Or in the settings. (System → Info)

But because otherwise it is not activated and you have a "trial". Actually, your key is linked to the mainboard, so for your new pc you probably need a new key.