Hi.
i checked the www Object action and it has this line :
#if !(UNITY_SWITCH || UNITY_TVOS || UNITY_IPHONE || UNITY_IOS || UNITY_ANDROID || UNITY_FLASH || UNITY_PS3 || UNITY_PS4 || UNITY_XBOXONE || UNITY_BLACKBERRY || UNITY_WP8 || UNITY_PSM || UNITY_WEBGL)
Which means that this does not work on these platforms.
This is a Unity limitation, not playmaker.
But i did a quick search on the
Ecosystem (searched 'url') and there is an action called 'Open Wbgl Url'