Rockstar Games provides specific channels for reporting third-party software like Terror:

Use of a Mod Menu (Terror) to gain an unfair advantage or disrupt gameplay.

Links to unlisted YouTube videos or Imgur screenshots are the most effective forms of proof.

How to report cheaters in Red Dead Online - Rockstar Support

[The name of the player using the menu]

Be aware that some high-end menus like Terror have "protections" that can sometimes block or intercept in-game reports, or even crash the reporter’s game. Using the external Rockstar Support webform is generally considered the most secure way to ensure your report is received.

While Rockstar primarily uses the webform, some issues can be flagged via their community resources at rockstargames.com/community-resources .