When running the command;
   /data merge entity @e[type=minecraft:trident,limit=1] {Passengers:[{id:"chicken"}]}
the targeted entity, the trident, does not receive a chicken passenger, despite being told that it "Modified entity data of Trident", however when you run;
   /summon trident ~ ~ ~ {Passengers:[{id:"chicken"}]}
you spawn a trident with a chicken riding it as expected.