Please subscribe by email instead:

Due to abuse, the web-based subscription has been disabled. You can join by sending email to the list's -join address instead.

It looks like you were trying to subscribe to openwrt-announce@lists.openwrt.org.

In that case you should be able to subscribe by sending a PLAIN TEXT (not HTML) mail containing only the single word subscribe to openwrt-announce-join@lists.openwrt.org.

It must be plain text, and it must go to the -join address, not to the list itself.