For others who stumble here, if you need to specify a variable in the URL string, this is called a “path” variable.
To initiate a path variable, when you first create the Operation, just beneath the Identifier, in the Path section, type /path/to/{myVar}. Once you put the “{myVar}” part, it will automatically create your Parameter for you. If you had initially create this variable, delete the variable first, then re-add the variable into the Path section.
Example:
Type in the path (in my example “{urlString}”):
Then check your Parameter to ensure the kind is “Path”:
Hope this helps.