Verify burp using scanner issues manually portswigger trigger payload highlighted tab issue request been used if Burp extension smuggling smuggler Using burp to manually verify scanner issues
Leveraging Burp Suite extension for finding HTTP Request Smuggling
Leveraging burp suite extension for finding http request smuggling

Using Burp to Manually Verify Scanner Issues - PortSwigger

Leveraging Burp Suite extension for finding HTTP Request Smuggling