Create a new domain name and set the A record, then set the listener on the server.

Thank you for quick reply, I dont know what you mean by create a new domain,.. you mean i should by new domain or i should create a new subdomain...?
And about set a listener i dont know how to do that... or i dont know the full meaning.. can you clarify what is or how to set a listener on server..?

Create a new subdomain.

And add configuration to your Nginx like this:

server {
 ...
        server_name www.94cb.com 94cb.com m.94cb.com api.94cb.com;
...
 }

What if i use Apache

after creating a subdomain what should i add in the folder

Apache:
add ServerAlias

<VirtualHost *:88>
DocumentRoot /www/web/carbon/public_html
ServerName 94cb.com
ServerAlias www.94cb.com m.94cb.com api.94cb.com

...

</VirtualHost>
<Directory /www/web/carbon>
    Options FollowSymLinks
    AllowOverride All
    Require all granted
</Directory>

1. Create a subdomain and set A record;
2. Set your subdomain information in your carbon forum dashboard;
3. Add ServerAlias to your apache and reload it.

so the subdomain will be empty... and is this going to work in shared host.

empty?

I ask you what should i add in subdomain and i think you didn't read that comment...
So far i know this step

1. I create a another subdomain in folder where i host carbon forum
2. I get to DNS the i create a A record with subdomain i create and i point to the same ip address
3. THIS STEP I DON'T UNDERSTAND, BECAUSE I SEE THE CODE YOU GIVE ME IS VPN NOT SHARED HOST ...?

Am Beginner, I found this forum so beautiful and i want to use it with our website. So i need step by step how to do this i know it will help other too so please help.

Shared host users should point your multiple subdomains to the same folder.

You mean, please am beginner can you please be specific PLEASE a little instruction for beginner will be very kind. PLEASE SIR

Please sir a little help.

I am not using a shared host at present, and I am not sure how to operate it on the shared host's user interface.

Can i give you credential in my shared host so that you can test it.