Typing in the tag {{/give @s minecraft:trial_key[minecraft:custom_name=

{"text":"Big Trial Key", "color": "dark_purple", "font": "minecraft:alt"}

]}} like you would in the /tellraw command will throw this error:

Malformed 'minecraft:custom_name' component: 'java.io.EOFException: End of input at line 1 column 3 path $.'

 However, typing in the same tag in Data Pack format works perfectly:

{   \"text\": \"Big Trial Key\",   \"color\": \"dark_purple\",   \"font\": \"minecraft:alt\" }