WARNING !
Had to realize that errors can occur when indenting. So it’s best to avoid it.

@venus642

It would be nice if Notepad++ could handle that.

It is Powershell syntax. Notepad++ cannot do anything about how Powershell works.

The Lexilla component code is:

		} else if (sc.state == SCE_POWERSHELL_HERE_STRING) {
			// This is a doubles quotes here-string
			if (sc.atLineStart && sc.ch == '\"' && sc.chNext == '@') {
				sc.Forward(2);
				sc.SetState(SCE_POWERSHELL_DEFAULT);
			}

sc.atLineStart is a condition to close the Here-string, it must be at the start of the line. It complies with the current syntax of Powershell.

If you want change to support indentation of Here-strings, then Powershell core development is where the change needs to done.

It would be nice if Notepad++ could handle that.

Looks to be the Powershell language.
Quote from Microsofts Powershell about_Quoting_Rules documentation on Here-strings.

In either format, the closing quotation mark must be the first character in the line.

So it appears that "@ needs to be at the start of the line to be valid syntax.
Powershell Here-strings are not designed for indenting. So you may need to break the implied rule of indenting by:

Yes, powershell problem…
Yes ugly, I would like it to be fully indented…

Looks to be the Powershell language.

Quote from Microsofts Powershell about_Quoting_Rules documentation on Here-strings.

In either format, the closing quotation mark must be the first character in the line.

So it appears that "@ needs to be at the start of the line to be valid syntax.

Powershell Here-strings are not designed for indenting. So you may need to break the implied rule of indenting by:

function test1
{
$test = @"
This is a test
"@
}

	function test2
	{
	$test = @"
This is a test
"@
	}

Looks ugly though it is valid syntax.

And you would change the setting for the language of interest (maybe what you show is Perl?).