The marketplace does not have a gift function. If you understand how marketplace transactions work, you can exploit it to give people gift cards they can use to buy items.
The marketplace does not bill your credit card directly, instead the store you brought the sim through does. This means if you brought the game through Steam, the Steam billing system handles the transactions for DLCs, if you used the Microsoft store to buy MSFS, then it is the XBOX billing system that handles the money.
With that in mind, if you want to buy a friend an addon, find out what platform the brough the sim from and buy them the appropriate gift card. I have tested this with Steam, but not with the Windows Store version. However just after launch I remember reading a post from someone who had used a Microsoft gift card (I am not sure if this was an Xbox or MS Account gift card).