Most of the harassment that Uighurs face overseas and in the diaspora is directed toward their family members back in China. Most Uighurs who are living abroad still have parents or close relatives back in China. Their online activity is being watched quite closely. Their family members will be contacted by the police if they are seen to be protesting or speaking up too loudly.
That has the effect of silencing Uighur voices in the diaspora. Their family members are in effect being held hostage and are being threatened with potential detention. Many people who are living abroad already have family members in detention in the camp system, but they might have one or two who aren't, so the threat remains that the rest could be taken.
That's the primary way that harassment happens. There are also efforts on the part of the Chinese state to convince Uighurs in the diaspora to collaborate with them and to monitor Uighurs abroad and then report on Uighur activities. We've seen this documented in numerous cases. That also has a chilling effect among the Uighurs in the diaspora because they feel they can't speak openly because they're still being watched by other Uighurs.
Those are the primary ways that harassment happens, as far as I've seen. Chinese agents might also be working and observing, but I don't have any details on that.