For use in a macOS app to launch 1Password and display all login items for a particular domain. Use onePasswordAvailable in Cocoa Bindings to enable or hide a UI element. Use onePasswordIcon as the image for the UI element.

Launch1PasswordSearch.swift

// set the domain in the search URL appropriately for your site
private let onePasswordSearchURL = URL(string: "onepassword://search/EXAMPLE.COM", relativeTo: nil)!

// bind this to the NSButton's Hidden value, with a Value Transformer of NSNegateBoolean
@objc dynamic var onePasswordAvailable : Bool {
    return (NSWorkspace.shared.urlForApplication(toOpen: onePasswordSearchURL) != nil)
}

// bind this to the NSButton's Image value
@objc dynamic var onePasswordIcon : NSImage? {
    guard let applicationURL = NSWorkspace.shared().urlForApplication(toOpen: onePasswordSearchURL) else { return nil }
    return NSWorkspace.shared.icon(forFile: applicationURL.path)
}

// bind the NSButton's action to this function
@IBAction func launchOnePasswordSearch(_ sender:AnyObject? = nil) {
    NSWorkspace.shared.open(onePasswordSearchURL)
}

I've confirmed with AgileBits that this is the best approach currently possible.