For the webhook subscriptions, you can only use the triggers available on the dropdown select list. The hs_lastmodifieddate cannot be used in this case. There are only two date picker properties available : closedate and create date as you can see in the screenshot below :
Finding an associated company to a deal would not be possible. Actually what you can do is to activate a webhook everytime there's an associated deal to a company. You can retrieve this by using the closedate property on the company object, like the screenshot below). That can maybe be a workaround.