It depends you can either :
Create you own banner just for the use on the forums and then edit this in paint for example with your subs / goals - and then paste this into the signature section of your forum account
Or you can use cyanlabs youtube signature generator - which will take the banner you have associated with your YouTube account, allow you to set sub / other goals and then provide you with a URL link which you just have to copy and paste into the signature section of your forum account