Idk why you would want to do this in the first place but with stuff like this (rocket jump/crouch jump scripts) it's done with two separate aliases with + and - so that it knows to do -attack when you let go of the key. Try this (except replace the key with b or whatever key you want) also if you are trying to do it for rocket jumping the script below won't really work (since it didn't have you crouch) I just didn't know what to call it.
alias +rj "+jump; +attack"
alias -rj "-attack; -jump"
bind key +rjalso the syntax of your code is kind of silly, you only need quotes around multiple commands at once (i.e., +attack; +duck or anything separated with spaces and semicolons) so "bind b" doesn't make much sense