Download the softphone installer from the official website at the link. Install it by following the instructions of the installer.
The following window will be displayed at the first launch:
Accept the agreement and click "Use SIP account"
In the "Username" field, enter the internal number of the employee
In the "SIP Domain" field, enter the IP address of the MikoPBX you are going to connect to
In the "Password" field, enter "SIP password"
Click "Use"
The green indicator on the top indicates successful authorization. Also, the status Online should be displayed on the PBX for this internal number.