Hi @yezo1,
You could configure From in "When a new email arrives" so that Flow will only trigger when it receives an email from a certain email address.
Then configure the Condition to determine if the body contains a certain string.
Configure Forward an email in if yes, and configure a specified address in “To” to send it to the specified person.
However, the subject cannot be changed in Forward an email, this feature is not supported currently.
Or you could configure Send an email directly, but this is not Forward an email.
Image reference:

Take a try and let me know if you have any issue.
Best Regards,
Barry