I found the answer, if any one else needs to configure multiple local BGP AS
config router bgp
config neighbor
edit "IP of the neighbor"
...
set local-as 300
set local-as-no-prepend disable|enable
set local-as-replace-as disable|enable
end
Enable local-as-no-prepend if you do not want to prepend local-as to incoming updates.
Enable local-as-replace-as to replace a real AS with local AS in outgoing updates.
