If it even works you could try:
#define SOMEMACROCOMMENT "This is a tooltip"
UPROPERTY(meta = (ToolTip = SOMEMACROCOMMENT))
float MyProperty = 1.f;
But I have never been in a situation I’d need a reusable comment. That might point at a bad code style. Also structs should just be used as data container for “any” purpose, as generic as the code allows. So it’s hard to add a single comment / tooltip to a struct.