PageRule Post Api Bad Request

I am using the following powershell to manipulate the api related to page rules. I can get, delete fine so I know my headers are set correctly, but can’t post to create a new page rule. All I get is bad request and no more information. I’ve removed my zone id from the following for privacy reasons.

$cloudFlareUrl = $cloudFlareBaseUrl + 'zones/' + $cloudFlareZoneId + '/pagerules/'

this makes: zone id/pagerules/
I’ve tried this with and without the trailing slash

$forwardingRule = '{targets:[{"target":"url","constraint":{"operator":"matches","value":"' + $sitePrefix + '.' + $SiteBaseUrl + 
    '/*"}}],actions:[{"id":"forwarding_url","value":{"url":"' + 
    $sitePrefix + '.' + $SiteBaseUrl + '/$1","status_code":302}}]}'
$result = Invoke-RestMethod -Method POST -Uri $cloudFlareUrl -Headers $cloudFlareHeaders -Body $forwardingRule

so forwarding rule is

I can set this up manually using the website no problem. I have set it up manually and also checked what I Http GET from the site is also what I am sending.

Please can we have more meaningful error messages? What is wrong with the request above?