tl;dr the Windows Sandbox Administrator must be enabled and blank passwords allowed. The password is blank.
Enable Administrator
By default, the Windows Sandbox Administrator account is disabled.
PS > Get-LocalUser -Name "Administrator"
Name Enabled Description
---- ------- -----------
Administrator False Built-in account for administering the computer/domain
To enable the Administrator account
net user "Administrator" /active:yes
Allow blank passwords
Then set policy Accounts: Limit local account use of blank passwords to console logon only to Disabled.
- Start
gpedit.msc
- Browse to Computer Configuration > Windows Settings > Security Settings > Local Policies > Security Options
- Open
Accounts: Limit local account use of blank passwords to console logon only
- Select Disabled
- click Apply and OK

runas Administrator using blank password
To test, start an elevated cmd.exe command using runas.exe. Use a blank password (just press Enter)
PS > runas.exe /user:Administrator C:\Windows\System32\cmd.exe
Enter the password for Administrator:
Attempting to start C:\Windows\System32\cmd.exe as user "3C3F3100-C191-4\Administrator" ...
In the new cmd.exe window, check the user
C:\Windows\system32>whoami.exe
3c3f3100-c191-4\administrator