Free Burp Collaborator Alternatives
The best one that you don’t have to host yourself is interact.sh by far. But I’ve seen that domain get blocked before on outbound traffic from targets. So you may want to use one of the others as a backup or setup your own server using the instructions on the interact.sh github.
Setup yourself:
https://github.com/projectdiscovery/interactsh
https://github.com/redfast00/malidate