You cannot directly send DLC from a PC to an Xbox player
The Xbox player must buy or redeem the DLC on their own Xbox account
If the DLC is a giftable item, buy a digital gift card and send it to them
If the game supports cross-platform entitlements, the DLC may carry over only through the same linked account
Make sure the Xbox player is signed in with the correct Microsoft account
Check whether the DLC is included in a game pass, edition upgrade, or bundle instead of sold separately
Use the Xbox Store on the console, Xbox app, or Microsoft Store to purchase the DLC for your own account only
If you want them to receive it, share the product code or gift code if you have one
Contact Xbox Support if the DLC was purchased on the wrong account