Be there or be square!
PowerShell, 59 62 61 60 bytes



$z=ls($d=$args)-di;('"+-+
| |
+-+"','mv b.ps1 "$d"')[$?]|iex


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Explanation

PowerShell's Move-Item cmdlet (mv) renames a file as well, so giving it a directory that doesn't exist as the destination just renames the file to that last component instead (as long as the parent exists), so that was no good.

I could use Test-Path to determine that the destination exists and is a directory, but it's too long Test-Path $d -PathT C.

So instead I'm using Get-ChildItem (ls) with the (shortened) -Directory parameter, and checking $? to see if it was successful. The output if there is any is assigned to $z so that it isn't seen.

That's done in the form of an array with 2 elements, then indexing into the array with the boolean value of $?, which will get coalesced to 0 or 1, so the first array element is chosen if the destination directory doesn't exist, and second if it does.

The first array element is a string containing the box (quoted); newlines are allowed in strings, even when they're not heredocs. The second element is a string containing the move command.

The result of that array selection gets piped into Invoke-Expression (iex) to be executed. This is necessary because of I just leave the actual move command in the array, it gets executed no matter what (to populate the array), which defeats the purpose.