I have created a wallet with Electrum. Now I want to get the public key (not the address, but the public key for key-derivation).
How can I achieve that?
If you mean the extended public key aka master public key then you can get that via wallet menu > information. If you mean the address specific public key go to view menu > show address, switch to addresses tab, right click on the address in question and choose details to view the public key. You can also search the addresses on that tab by pressing ctrl+f.