https://www.fbi.gov/investigate/terrorism/tsc
The TSC works to protect privacy and civil liberties at all steps in the watchlisting process. To add a person to the watchlist, specific intelligence-related criteria must be met.
Only government agencies can nominate someone to the terrorism watchlist. You cannot nominate someone you know.
No one can be added to the watchlist based on:
Race
Ethnicity
Religion
Beliefs or activities protected by the First Amendment
Guesses or hunches