Json Formatting issue sending over http post request

[TobyNorthrup]

I am using AT commands to send a post request using application/Json. I am trying to Format my post request to send multiple values for every key but I having trouble getting the formatting correct so that I don't get an error

This is my serial output, which is showing how I have it formatted to send now

This is how I am formatting it now in my code

How can I format the string so that I can send multiple values for every key { "Lat": [val1, val2 ,val3], Long: [val1, val2, val3], ....}

I have tried to updated it but I can't get it to format correctly