Using device MAC address as MQTT topic

Hello everyone,

Could someone help me please to get this right?
I want clients connected to MOSQUITTO to listen their own "personal" topics and the best solution I found is to communicate physical addresses to MOSQUITTO. Later MOSQUITTO would publish data in each topic.
So far I manged to communicate address using the following:

String clientId = WiFi.macAddress();

client.publish("/newClient", clientId.c_str());

Now...
Ho do I subscribe to topic with the name matching WiFi.macAddress()
Thank you.

Forgot to mention that it is an isolated home environment and I am not concerned about broadcasting MAC addresses.

For some reason this solution didn't work at the moment when the post was created:
client.subscribe(clientId.c_str());

After code has been cleaned up and reviewed I tried again and it worked.
Thank you.

For some reason this solution didn’t work at the moment when the post was created:
client.subscribe(clientId.c_str());

It did work. It did exactly what you told it to do. That it did not accomplish what you wanted is all we can glean from your useless statement. We have no idea what it actually did, or what you expected it to do. We have no proof that your mosquito actually send any message like what you tried to subscribe too.

PaulS:
we can glean from your useless statement.

Thank you Paul, your answer is very helpful and supportive. This attitude attracts a number of enthusiasts to this community.

I didn't ask anyone to rewrite the code for me, I didn't ask anyone to commit hours to deal with my issue. All I was looking for - a command that calls specific function as a confirmation that the error is somewhere else. So I could go back and correct what I have done wrong.
No one "except you of course" confirmed the right function. Now when I am confident this function works I decided to share this knowledge with the community just in case someone else who is also new doesn't even know what / how to ask can find the answer - you are making "helpful statement".
Thank you and all the best.