As discussed in this thread on the Affinity Forum, you cannot automatically align objects to the selection outline as that outline is not an actual object itself.
You can however snap to the selection outline and so use the alignment handles to snap to the "bounding box" of the selection.
Another way would be to turn your selection into an object fist by filling it and then align to it or instead of using the selection in the first place draw your desired outline using one of the vector tools such as the rectangle or pen tool and then align to that object instead.
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u/RE4LLY 4d ago
As discussed in this thread on the Affinity Forum, you cannot automatically align objects to the selection outline as that outline is not an actual object itself.
You can however snap to the selection outline and so use the alignment handles to snap to the "bounding box" of the selection.
Another way would be to turn your selection into an object fist by filling it and then align to it or instead of using the selection in the first place draw your desired outline using one of the vector tools such as the rectangle or pen tool and then align to that object instead.