Create the file $HOME/.smb/smb.conf with the following contents:

[global]
    workgroup = MYWORKGROUP

Taken from the libsmbclient documentation:

When libsmbclient is invoked by an application it searches for a directory called .smb in the $HOME directory that is specified in the users shell environment. It then searches for a file called smb.conf which, if present, will fully over-ride the system /etc/samba/smb.conf file. If instead libsmbclient finds a file called ~/.smb/smb.conf.append, it will read the system /etc/samba/smb.conf and then append the contents of the ~/.smb/smb.conf.append to it.
libsmbclient will check the users shell environment for the USER parameter and will use its value when if the user parameter was not included in the URL.