I would just use the GUI:
Firewall/NAT tab
Firewall Policies subtab
+ Add Ruleset in the top left corner
Name: whatever you like
Default Action: Accept
Save
then on your newly created ruleset:
Actions on the right hand side
Interfaces
choose the interface you want to apply it to
for direction I guess i would pick out, since you are trying to filter this ICMP redirect traffic coming out of that interface...
Save Ruleset in the bottom right
