This issue runs afoul of free speech, until such time as the designated group actually acts as terrorists.
The idea is, on its face, unconstitutional, and is extremely dangerous. Consider that the Southern Poverty Law Center (SPLC) will likely be the preferred source for terrorist designation - and they have already designated just about every conservative leaning group in the nation as terrorists, including Dobson’s “Focus on the Family” group (forgive me if my memory of the group name is faulty).
BTTT...
You can be sure that the day after it’s signed FR will be designated a terrorist group, and anyone who ever donated twenty bucks online will have a SWAT team at the door.