You can manually add custom services to the drop-down menu. Custom services will appear at the top of the menu.

var a2a_config = a2a_config || {};
a2a_config.custom_services = [
	["Example dot com",
	 	"https://www.example.com/add?url=A2A_LINKURL_ENC&title=A2A_LINKNAME_ENC",
		"https://www.example.com/images/icon_32x32.png",
	],
	["Example dot net",
	 	"https://www.example.net/add?url=A2A_LINKURL_ENC&title=A2A_LINKNAME_ENC",
		"https://www.example.net/images/icon_32x32.png",
	],
	["Example dot org",
	 	"https://www.example.org/add?url=A2A_LINKURL_ENC&title=A2A_LINKNAME_ENC",
		"https://www.example.org/images/icon_32x32.png",
	],
];

In Drupal, add the following JavaScript code to your "Additional JavaScript" box in Configuration > Web services > AddToAny > Additional Options.

In WordPress, add the following JavaScript code to your "Additional JavaScript" box in Settings > AddToAny.

a2a_config.custom_services = [
	["Example dot com",
	 	"https://www.example.com/add?url=A2A_LINKURL_ENC&title=A2A_LINKNAME_ENC",
		"https://www.example.com/images/icon_32x32.png",
	],
	["Example dot net",
	 	"https://www.example.net/add?url=A2A_LINKURL_ENC&title=A2A_LINKNAME_ENC",
		"https://www.example.net/images/icon_32x32.png",
	],
	["Example dot org",
	 	"https://www.example.org/add?url=A2A_LINKURL_ENC&title=A2A_LINKNAME_ENC",
		"https://www.example.org/images/icon_32x32.png",
	],
];