NSService

AppDelegate.m

@implementation AppDelegate

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
    [NSApp setServicesProvider:[MyService new]];

...

content.h

@interface MyService : NSObject
@end

content.m

@implementation MyService

- (void)processService:(NSPasteboard *)pboard userData:(NSString *)userData error:(NSString **)error {
    NSArray *classes = [NSArray arrayWithObject:[NSString class]];
    NSDictionary *options = [NSDictionary dictionary];
    
    if (![pboard canReadObjectForClasses:classes options:options]) {
        *error = NSLocalizedString(@"Error: couldn't encrypt text.", @"pboard couldn't give string.");
        return;
    }
    
    // Handle your content
    [pboard clearContents];
}

@end

Info.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<array>
	<dict>
		<key>NSMenuItem</key>
		<dict>
			<key>default</key>
			<string>Archive with My Application...</string>
		</dict>
		<key>NSMessage</key>
		<string>processService</string>
		<key>NSPortName</key>
		<string>MyService</string>
		<key>NSRequiredContext</key>
		<array/>
		<key>NSSendFileTypes</key>
		<array>
			<string>public.item</string>
		</array>
		<key>NSSendTypes</key>
		<array>
			<string>NSStringPboardType</string>
		</array>
		<key>NSServiceDescription</key>
		<string>Process your files with My Application</string>
	</dict>
</array>
</plist>