Cross platform is still missing a lot of features. This will have to get added with cross-save once DE finishes it. Along with stuff like cross platform trading, gifting, and clans.
For now you just have to deal with manually inviting your friend to your game or vice versa and being locked out of a lot of other social functions with them. Their name will start to auto-fill in the manual invite text box after you've invited them a few times.
Side note If there's an account on your platform than has the same name as one on a different platform, attempting to send a gift cross-platform will end up sending it to the same-platform account with the identical name (It will ignore the number id for some reason). So don't even touch gifting until that's sorted out. It'll probably still have issues for a while after the cross save update as they iron things out.
PollexMessier's post in I'm playing on Ps4.. i got a friend from PC but i can't send him a friend request... How can i do? was marked as the answer