“Exposed on Spawn” is only relevant in BP.
In C you would set the values as you would any other property. If they’re public, you can just set them. If they’re private you can use a Setter method.
UMyWidgetClass* MyNewWidget = T<UMyWidgetClass>CreateWidget(Owner, MyWidgetClassPtr);
if (MyNewWidget) {
//set public variable
MyNewWidget->MyPublicFloat = 1.0f;
//set private variable using setter method defined in your widget class
MyNewWidget->SetMyPrivateFloat(5.0f);
}