I think I found it. MapPropertiesTree => Players => [2] => ReservedHeroes => Right-click => Add. But not "New," instead, a menu called "AdvMapHero" opens in this line via the three dots. In it, under Maps/Multiplayer/XL2, I selected the only line in this directory (logically, it should be the map with the largest size, and there's probably a reference to that here). Then, a window with the hero's properties appeared on the right. I'll try to configure all the parameters to match the hero placed on the map and will report the result – whether the map will crash or not, or whether it will be possible to summon him from the reserve with the specified name.

Update: It doesn't work. Perhaps I'm not filling something out correctly. After exiting the map, it doesn't save in ReservedHeroes for some reason.