What are the JSON field format i need to trigger the survey from an external system?

I am trying to trigger a survey from an external system.
I have created a simple json file that is sending
{
"Title": "JSONTest",
"FirstName": "Bob",
"LastName": "Smith",
"PostCode": "L1 1AA",
"Email": "morgan@test.co.uk",
"Age": "36",
"DoB": "1980-01-01"
}
To the URL copied from the Action - JSON trigger.
I am authenticating using basic and that seems to get through as I am now getting a 400 - not a json format. I think it is something to do with the structure of the JSON fields in the Survey Action set up. I want to use all these as "string".
How do i structure this fields? I can;t seem to find out exactly on any support page? do i use something like the $.object.string or a field name?
Any help would be great! Thank you!