In Bannerlord beta e1.3.0 copy/paste was removed from the banner editor. Probably because it could crash the game. I created a small mod available here: BannerPaste that allows you to copy/paste like before. As a bonus it likely won't crash if you paste something invalid.
Make your banner. Copy it using the button above. Press Ctrl+V in the Banner Editor
Note that pasting with Ctrl+V will crash your game if you paste something that isn't a valid banner code.
You can change your banner by pressing B on the campaign map, or by clicking the banner in the top left of the Clan screen.