Here is a list of useful information for new Yeastar Cloud PBX users:

Linkus User guide: Linkus Desktop Client Guide (yeastar.com)
Yeastar Cloud PBX user guide: Cloud PBX user guide (Unofficial) (yeastar.sg)
Linkus download: Linkus Download and Documents – Linkus Download and Documents

Other information:
The user welcome email is sent by your admin, and the links and QR code in it is only valid for 24 hours. If you need it to be sent to you again, please look for your Yeastar Cloud PBX’s admin and ask to send yourself a new welcome email.