How do I install a font from the Windows command prompt?

It's possible but you have to write a Windows shell script to do that. Copying alone won't install the font: you also need to register the font, e.g.

copy "FontName.ttf" "%WINDIR%\Fonts"
reg add "HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Fonts" /v "FontName (TrueType)" /t REG_SZ /d FontName.ttf /f

Alternatively you can the following lines of code to suit your needs; save it as a .vbs file and then execute it.

Set objShell = CreateObject("Shell.Application")
Set objFolder = objShell.Namespace("<Folder or Share Location>")
Set objFolderItem = objFolder.ParseName("<TTF File Name>")
objFolderItem.InvokeVerb("Install")

Example:

Set objShell = CreateObject("Shell.Application")
Set objFolder = objShell.Namespace("C:\Windows\Font")
Set objFolderItem = objFolder.ParseName("Myriad Pro.ttf")
objFolderItem.InvokeVerb("Install")

Yet another alternative is to install fonts "temporary", just for current user session. The idea is to run fontview.exe for each font, which makes it available for other Windows applications:

for /F "delims=;" %%a in ('dir C:\ExtraFonts /B /A-D-H-S /S') do fontview %%a

See the complete solution here.

In Powershell this can be as simple as:

$fonts = (New-Object -ComObject Shell.Application).Namespace(0x14)
dir fonts/*.ttf | %{ $fonts.CopyHere($_.fullname) }

Similar to GeneQ's solution, here is a version doing it for all .ttf files in the script's directory:

Set ofso = CreateObject("Scripting.FileSystemObject")
SourceFolder = ofso.GetParentFolderName(Wscript.ScriptFullName)

Const FONTS = &H14&

Set objShell  = CreateObject("Shell.Application")
Set oSource   = objShell.Namespace(SourceFolder)
Set oWinFonts = objShell.Namespace(FONTS)

' Lame VBscript needs 4 f*ing lines instead of "if (/\.ttf$/i) " ...
Set rxTTF = New RegExp
rxTTF.IgnoreCase = True
rxTTF.Pattern = "\.ttf$"

FOR EACH FontFile IN oSource.Items()
    IF rxTTF.Test(FontFile.Path) THEN   
        oWinFonts.CopyHere FontFile.Path
    END IF
NEXT

You can also use the FontReg utility to install fonts from a command prompt.