Sysadmin rights to local administrator
I had to reread the vendor requirements a couple of times, but I take the instructions to mean the local administrator account only requires elevated permissions during installation.  Run one of the following blocks prior to the app install to set this up.

-- Add the Local Administrators Group
USE [master]
GO
CREATE LOGIN [BUILTIN\Administrators] FROM WINDOWS WITH DEFAULT_DATABASE=[master]
GO
ALTER SERVER ROLE [sysadmin] ADD MEMBER [BUILTIN\Administrators]
GO

-- Or Add only the Local Administrator User - Dynamic SQL to capture servername
DECLARE @hostname NVARCHAR(128), @createStmt NVARCHAR(MAX)
SELECT @hostname = CAST(SERVERPROPERTY(N'ComputerNamePhysicalNetBIOS') AS NVARCHAR(128))

SET @createStmt = N'USE [master];' + CHAR(13) + CHAR(10) +
    N'CREATE LOGIN [' + @hostname + N'\Administrator] FROM WINDOWS WITH DEFAULT_DATABASE=[master];' + CHAR(13) + CHAR(10) +
    N'ALTER SERVER ROLE [sysadmin] ADD MEMBER [' + @hostname + N'\Administrator];'

PRINT @createStmt
EXECUTE (@createStmt)
GO


When the installation is complete, be sure to back out these permissions as they are not the most secure, by running one of the following:

-- Drop Local Administrators group
USE [master]
GO
DROP LOGIN [BUILTIN\Administrators]
GO

-- Or Drop the explicitly defined Local Administrator login
DECLARE @hostname NVARCHAR(128), @createStmt NVARCHAR(MAX)
SELECT @hostname = CAST(SERVERPROPERTY(N'ComputerNamePhysicalNetBIOS') AS NVARCHAR(128))

SET @createStmt = N'USE [master];' + CHAR(13) + CHAR(10) +
    N'DROP LOGIN [' + @hostname + N'\Administrator];'

PRINT @createStmt
EXECUTE (@createStmt)
GO