The method attribute is used if the script is a method of the frame (usually added to the frame via a mixin attribute included in the <Frame ...> tag).
Because your function is a global (and so long as the .lua file is listed in the .toc before the xml file) you would just call the function:
Lua Code:
<Scripts>
<OnLoad>
AddonParent_OnLoad()
</OnLoad>
</Scripts>