Firstly, other than as noted in the cross-platform notes. most things just work on iOS and Mac - particularly anything that internal to Drafts, like manipulating text and drafts.
Second, you can start with the Insert Date example action to show how to insert text in the current draft. It takes a template, in the example case to insert the current date. If you change the template in the insert text step in this example from [[date|%B %d, %Y]] to [[created|%B %d, %Y]], it will insert the created date of the current draft.
If you want the date/time in other formats, take a look at the notes is the template guide.
Lastly, not sure what your use-case is, but, generally speaking, if you are sending the draft somewhere and want the creation date to go along, it might be better to insert it with the template for whatever export action you are using, rather than actually adding it to the content of the draft.
Hope this helps!