No. A physical Xbox 360 or Xbox One disc copy does not entitle to a separate digital key or PC license.
For Xbox Play Anywhere titles, the digital license that works across Xbox and Windows is tied to a qualifying digital purchase of the game from a Store or via a digital code, not to ownership of a physical disc. The usage rules state that Xbox Play Anywhere games can be purchased in a Store or via a digital code at participating retailers, and then installed on compatible Windows devices and Xbox consoles, but they do not provide any mechanism to convert a disc into a PC entitlement or to issue a free PC key based on disc ownership.
Since Forza Horizon 3 has been delisted from Microsoft Store and keys are no longer normally sold, there is no supported way to obtain a new PC digital license from Microsoft using only the physical Xbox copy.
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