Newbie parameter trouble Alamo5.2

Hello Fellow Swift Folks,
I am trying what seems to be something rather simple. To use the parameters to pass in the POST body. So this is the JSON I would like to pass:

{
"searchCriteria": {
"dateFilters": [
{
"dateType": "sentTime",
"fromDate": "2011-03-19T12:27:31.304+0000",
"toDate": "2020-03-21T12:25:31.304+0000"
}
]
}
}

And this is what I'd like to try to format with:

let parameters: [String: String:[String:String]] = [
"searchCriteria":
"dateFilters: [
"dateTime":"sentTime",
"fromDate":"2011-03-19T12:27:31.304+0000",
"toDate":"2020-03-21T12:25:31.304+0000"
]
]

I can't quite see why this isn't right. I can do this and it seems fine, syntax-wise:
// this works
let parameters: [String:[String:String]] = [
//"searchCriteria":
"dateFilters": [
"dateType": "sentTime",
"fromDate": "2011-03-19T12:27:31.304+0000",
"toDate": "2020-03-21T12:25:31.304+0000"
]
]
It seems by having another String "searchCriteria": I can't get the format right.
Better yet, given my original JSON strings which I'd like to send, is there a function or tool I can use to create the 'parameters template' I am trying to make?
Thanks,
Doug