You have exactly 2 options:
Either you contact an admin on the Teamspeak server directly and he sets it to permanent at your request, or you create a semi-permanent channel yourself and write in the channel description “Make me permanent”
Then your channel will be moved to the private user channel area.
Please note the following:
If you create a temporary channel and you write "Make me permanent" in the description, this will be ignored.
If you do not use your private channel for more than 2 weeks, it will be automatically deleted from the system and you will have to request it again.