You gotta avoid pasting out of “rich” apps, which includes the ISE I think. 
They end up pasting HTML. That’s where your extra SPAN tag came from in your post. LMK what you pasted from – I’d like to post a clearer warning about that for other folks. You can also paste into the TEXT tab of the editor, which makes it a lot clearer what’s happening, instead of the VISUAL editor tab.
So, you may have things a bit mixed up. I’m not sure, so I’ll just cover all the bits.
First, YOUR parameter could be $fred. It doesn’t NEED to be $ComputerName. You only use $ComputerName because that’s consistent with the way the shell works. I just want to make sure you’re clear on that. This is also legal:
param([string]$purple) get-wmiobject -class win32_bios -computername $purple
There doesn’t need to be a “match.”
Also, your code isn’t quite set up well to work against multiple computers – it will, but it’ll be harder to capture errors.
param([string[]]$computername) foreach ($bozo in $computername) $a = Get-WmiObject Win32_OperatingSystem -ComputerName $bozo }
Would be a better pattern.
You cannot just tack on a -ComputerName parameter to a command – like Set-ItemProperty – which does not natively support that parameter. You’re correct on that. Just because YOUR script has a -ComputerName parameter doesn’t confer the ability to any command contained within the script.