Just define a url as follows:

<a class=” btn-primary btn-lg” href=”itms-services://?action=download-manifest&url=https://abc.com.tr/manifest.plist“>Download App</a>

and manifest.plist file should look like as follows:

<?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">
<dict>
    <key>items</key>
    <array>
        <dict>
            <key>assets</key>
            <array>
                <dict>
                    <key>kind</key>
                    <string>software-package</string>
                    <key>url</key>
                    <string>https://abc.com.tr/xxx.ipa</string>
                </dict>
            </array>
            <key>metadata</key>
            <dict>
                <key>bundle-identifier</key>
                <string>com.def.abc</string>
                <key>bundle-version</key>
                <string>0.10.0</string>
                <key>kind</key>
                <string>software</string>
                <key>title</key>
                <string>XXX</string>
            </dict>
        </dict>
    </array>
</dict>
</plist>

As you can see ipa file path is defined in manifest file.