If your component exists on the same GameObject as the SkeletonAnimation or has it as one of its child GameObjects, it's as simple as:
[SpineAnimation]
public string walkAnimation;
If it's on a different GameObject, you can define it like this:
public SkeletonDataAsset skeletonDataAsset;
[SpineAnimation(dataField:"skeletonDataAsset")]
public string walkAnimation;
Otherwise, just implement the IHasSkeletonDataAsset interface on your script and point the property to a SkeletonDataAsset. Then the [SpineAnimation] attribute will use the IHasSkeletonDataAsset interface automatically. Works like the first example.