How do we format parameters for Azure Policy assignment in PowerShell?

$subscription_id = "/subscriptions/blah"

$policy = Get-Content -Raw -Path "C:\Users\user\Desktop\DS_RG_Name_Enforcement.json"
$param = Get-Content -Raw -Path "C:\Users\user\Desktop\DS_RG_Name_Enforcement_Param.json"

$arr = ("resourceGroups","subscriptions/resourceGroups")
$map = @{'resourceTypes'=($arr)}

function Create-Policy {
    param([string]$name, [string]$subscription)
	
	$definition = New-AzPolicyDefinition -Name $name -DisplayName $name -Policy $policy -Parameter $param
    New-AzPolicyAssignment -Name $name -DisplayName $name -Scope $subscription -PolicyDefinition $definition -resourceTypes $arr
}

$res = Create-Policy -name "DS RG Name Enforcement2" -subscription $subscription_id

I am currently using the above to create and assign a policy to a subscription. However the parameters are not being set correctly when the deployment is finished. I am trying to target "resourceGroups" and "subscriptions/resourceGroups" under Microsoft.Resources in the edit assignment window, but after every deployment the parameter has 0 selected

Any help here would be greatly appreciated. Thanks!!!