Hi,
For setting up custom addons in path your server, you can follow this video explaining the same:  How To Setup Custom Addons Path In Odoo Server

Thanks

I'm facing the same problem but I need another solution Yenthe 

Your way will work because both folders under odoo13 folder

But I need to make addons folders out odoo13 folder, I need it to be shared between more than one user to make one addons directory for more than one instance in the same time

I created a folder by the root user so he is the owner and I make group (sudo= which includes users of installed instances) is the owner group

Also I gave the folder and its sub-folders 775 permission

But unfortunately after uploading the module and restarting odoo service, it doesn't appear 

I faced the same problem. In my case, it was because the folder was empty. I didn't created the first module with the minimum file (the "__manifest__.py") in the module folder.