You can’t anchor a given process line to the top but
htop can follow a process if you press Shift+F. From the man page:
F "Follow" process: if the sort order causes the currently selected process to move in the list, make the selection bar follow it. This is useful for monitoring a process: this way, you can keep a process always visible on screen. When a movement key is used, "follow" loses effect.
Here I’m following my Firefox process:
Note that if you’re only interested in one particular process, the
-p <pid> will do what you want (12352 is the pid of Firefox on my system):
$ htop -p 12352