hi
I want to set in the Yast Bootsettings a new boot partition.
It should be an USB Stick, which isn’t permanently connected to the PC.
So if it is connected, it should boot from the stick, if not it should give an error.
If you search this sub-forum on my user name and “USB” you’ll find IIRC 2 threads with instructions on fully independent setups with a USB external and using a USB stick.
It is important to understand that the stick, while being a USB storage device like USB disks, is not identical technically. This can result in different behavior as a boot device. Furthermore, how USB boot devices are handled by the bios, is entirely dependent on how that particular bios is written. On some machines, with the USB stick configured as the first boot device, when the stick is not in the machine the bios will drop to the next configured device. But on another machine, the bios will not and the system will just hang; in this situation, the boot device configuration must be toggled in the bios back to a bootable drive. I have not found a way of predicting what a bios will do without actually testing on that specific bios.
Maybe. 
In that thread, the user wanted to put only the boot files on the USB stick. At first I thought OP wanted to have the whole OS on the stick; I misunderstood. So the method used in that thread is different - there we create only a /boot partition on the stick. The openSUSE root is actually in a partition on the internal disk. So OP is using the stick just to control the boot. (It’s important to read that whole thread.)
But if you want to install all of openSUSE on the USB device, then this thread is better How to boot XP first instead of SUSE?. Jump from the first post down to #18 and read all the way to the end, where the OP wrote his own howto.
fyi, there is more general info here No boot - 8.10 installed on external USB HDD in a post from this morning. This is just an overview of different methods, not the details like in the above.