kron occurrence backup at 0:00 Thu recurring policy-list backup ! kron policy-list backup cli write cli show running-config | redirect tftp://192.168.1.38/brtgate.config !
This creates a batch of commands named "backup" [where in typical ISO fashion everything is referred to as a "policy"] that will be executed every Thursday morning. This batch commits the running configuration to flash memory ["cli write"] and copies the running configuration to the specified TFTP server ["cli show running-config | redirect tftp://192.168.1.38/brtgate.config"]. The rather odd looking use of "redirect" is because the IOS "copy" command is interactive and interactive commands cannot be run via "kron".
Remember that the file on the TFTP server has to exist, even if zero sized, and be world writable; otherwise the redirect will fail with a permission denied error.