You're supposed to be able to set the address in the HP ThinPro registry and that value should be passed on to the ThinLinc client. However that doesn't work reliably right now.
The only time it works is when both username and password were also set. Most likely because we only tested it on the Zero versions of ThinPro where all fields are always populated.
The bug is simply poor variable handling where we will send empty arguments for the username and password to tlclient when they are not set. The first of these will then be interpreted as the servername.