the manual offers:
-k [ num ]
Read only one (or num) characters. All are assigned to the first name, without word splitting. This flag is ignored when -q is present. Input
is read from the terminal unless one of -u or -p is present. This option may also be used within zle widgets.
Note that despite the mnemonic `key' this option does read full characters, which may consist of multiple bytes if the option MULTIBYTE is set.