You are going to have to explain a bit more what you mean by "This [doesn't] seem to work". There is not much going on there. sed will place the string: xxxx://www.xxx.com/xxx/xx/xxxx/list/ in front of every line and edit the file list .
You might need to explain your expectation and maybe show some bits of the result.
For troubleshooting you might want to test as:
echo "^This was the beginning before" | sed 's|^|xxxx://www.xxx.com/xxx/xx/xxxx/list/|'
xxxx://www.xxx.com/xxx/xx/xxxx/list/^This was the beginning before