Ran, this is not working. It isn't parsing the URL correctly. When you copy from OneNote 2016, OneNote gives you two URLs at the same time and it is on two rows.
https://onedrive.live.com/view.aspx?resid=677C55B0601B8517%2166245&id=documents&wd=target%28Inbox.one%7C6458FF37-5C6C-7445-B468-11C74D2F77D7%2FSong%7C4A02E6C4-3273-5841-9DB4-B24A41C9CB99%2F%29
onenote:https://d.docs.live.net/677c55b0601b8517/Ed's%20Personal%20O365/Inbox.one#Song%20§ion-id={6458FF37-5C6C-7445-B468-11C74D2F77D7}&page-id={4A02E6C4-3273-5841-9DB4-B24A41C9CB99}&end
The first URL starts with the HTTPS:// url, and the second starts with onenote:https://
Normally the app you are pasting in only accepts one URL and discards the other. So if I paste this into Excel, it only accepted the https:// link and discarded the onenote: link.
If I paste it in OneNote, it only accpts the onenote: link and discards the HTTPS:// link.
If I paste it in Planner, it accepts both links and then says it isn't a valid link. Well of course it isn't. It is two links and it should not accept the onenote: link.
The only fix is to paste the links into notepad first, delete the onenote: row, then paste just the https: link.
Note that this used to work before this thread got started. I've been pasting OneNote links into Planner since it was in beta in 2016 I think, at least when it started accepting links. But whatever changed in Q3 or so of 2018 it quit looking at what was
getting pasted in and just accepted the whole thing from OneNote.
I have tried this with two different Office 365 tenants and both Planners say it is an invalid URL.