How do I block user-agent via iptables?

I'm trying to block a specific user-agent from all ports but it's not working.

I tried something like this.

Code:

iptables -A INPUT -p tcp --dport 80 -m string --algo bm --string "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:75.0) Gecko/20100101 Firefox/75.0" -j DROP

Can't find any info on this and stackoverflow just shits on simple questions.

by ZX3 via TBN