How Can I Type A Checkmark (✓) Via Alt Code?

The easiest way is to press Windows+. (dot) or Windows+; (semicolon) then type "check" to search for "check mark". This works anywhere on Windows, even without a numpad

See How do I bring up the emoji IME on Windows 10?

If you want to type arbitrary Unicode characters not in the above list then you must use the hex numpad by creating a registry key named EnableHexNumpad with type REG_SZ in HKCU\Control Panel\Input Method, set its value to 1 then reboot. After that you can type the character as Alt+➕Unicode codepoint value. The first numpad + before the code point is required!!!

So to type ✓ which is U+2713 just press Alt+➕2713

Similarly to type U+2705 ✅ press Alt+➕2705

See also:

• How to enter Unicode characters in Microsoft Windows
• Insert ASCII or Unicode Latin-based symbols and characters

Note that you can't use Alt code with decimal numbers larger than 255, except in a few applications that capture the Alt hotkeys and process the Alt codes themselves. Most will just use the default Windows behavior which is the integer value modulo 256 (i.e. the last byte of the value). As you already realized, the bullet appeared over and over again every 256 entries

Microsoft Word is one of the programs that treat Alt codes specially, that's why you see the above behavior. Alt+X is a special shortcut of MS Word and other similar text editors like Wordpad or Libre Office so of course you can't use it elsewhere. See I can use Alt-X to enter Unicode on an old machine. How is this possible?

For example 10003 % 256 = 19 (0x2713 & 0xFF = 0x13) and 10004 % 256 = 20 (0x2714 & 0xFF = 0x14). Look at code page 437 which is the default on most US computers and you'll see that 19/0x13 and 20/0x14 are code points for ‼ and ¶

Notepad++

Workaround 1

First workaround is to create/save a python script. Then run it when you need to insert the checkmark character.

1. Go to Plugins -> PLugins Admin. Install Python Script plugin
2. After the installation, Go to Plugins -> Plugins Admin -> Python Script -> New Script. Save window will open, give the script a name.
3. Enter the code below, then save it.

checkmark = u'\u2705'
editor.addText(checkmark)

4. When you want to insert the checkmark character, go to Plugins -> Plugins Admin -> Python Script -> Scripts, and select the script you saved. It will be automatically inserted

Workaround 2

Another workaround is to use find and replace to insert the checkmark character.

• Press Ctrl+H
• Find what: (a space)
• Replace with: \x{2705}
• Search mode: Regular expression

MS Word, LibreOffice Writer, Wordpad

Entering 2705 then pressing Alt+X works with MS Word (as already stated by @phuclv); it also works the same in LibreOffice Writer and Wordpad

MS Word

LibreOffice Writer