OK I worked this out
Go to Invite Guest user
Select the link for Bulk Invite
Download the bulk invite template
Set the send invitation field to FALSE
Fill out the other fields (redirect URL field needs the start http(s)//: get an error just doing www.....)
Upload the file and hit the submit button
Go and do all your setup over in PowerApps
Update the bulk invite file, set the Send Invitation field to TRUE
reimport the file and the submit button.