You macro looks good, just the IF statement is wrong. You need to check for a substring. This works:
{
"Name": "string",
"CreationDate": "2020-7-14",
"Commands": [
{
"Command": "executeScript",
"Target": "return (window.location.href)",
"Value": "u"
},
{
"Command": "if_v2",
"Target": "${u}.includes (\"wikipedia.org\") == true",
"Value": ""
},
{
"Command": "storeText",
"Target": "id=firstHeading",
"Value": "Heading"
},
{
"Command": "else",
"Target": "",
"Value": ""
},
{
"Command": "echo",
"Target": "Not Wikipedia",
"Value": "#shownotification"
},
{
"Command": "end",
"Target": "",
"Value": ""
}
]
}