Select a specific item in a dict SQL

Question

How do i select an item from a dict with sql? Im inserting on my table the following:

{"url":"https://gcphone.nyc3.digitaloceanspaces.com/aaef9495-90dc-48ec-b913-0902f7f8759b.jpg"}

But i want to create a trigger that before insert it only inserts:

https://gcphone.nyc3.digitaloceanspaces.com/aaef9495-90dc-48ec-b913-0902f7f8759b.jpg

How can i do that? Thanks