POST request in PowerShell with InvokeRestMethod

I Want to make a Post request in InvokeRestMethod.

Below is the body parameter i used to post content to Body

Expected parameter type is string for databaseName,password,roleName,databaseName.
Expected parameter type is stringarray for roles
My declaration for roles is not in stringarray type , it is being passed as json type i guess. please help in resolving below errors
$body = @{
databaseName='admin'
password= 'changeme123'
roles = {databaseName="sales";roleName="read"} | Out-String
username= 'sampleuser'
}
$json = $body | ConvertTo-Json
Invoke-RestMethod -Method POST -Uri $getProjectUri -Headers @{Authorization = "Basic $base64AuthInfo"} -Credential $credential -ContentType "application/json" -Body $json
Error: Invoke-RestMethod : {"detail":"Received JSON for the roles attribute does not match expected format.","error":400,"errorCode":"INVALID_JSON_ATTRIBUTE","parameters":["roles"],"reason":"Bad Request"} At C:\Users\xxxxxx\powershellscript\getusers2.ps1:40 char:3 + Invoke-RestMethod -Method POST -Uri $getProjectUri -Headers @{Autho ... + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + CategoryInfo : InvalidOperation: (System.Net.HttpWebRequest:HttpWebRequest) [Invoke-RestMethod], WebException + FullyQualifiedErrorId : WebCmdletWebResponseException,Microsoft.PowerShell.Commands.InvokeRestMethodCommand
 

I want to add users to database with specific roles , and am trying to convert below code from curl into InvokeRestMethod.

--request POST "https://cloud.mongodb.com/api/atlas/v1.0/groups/{PROJECT-ID}/databaseUsers" \
     --data '
       {
         "databaseName": "admin",
         "password": "changeme123",
         "roles": [{
           "databaseName": "sales",
           "roleName": "readWrite"
         }, {
           "databaseName": "marketing",
           "roleName": "read"
         }],
         "username": "david"
       }'