If you go every four or six weeks, the reliable way to keep your slot is to book the next appointment before you leave. The professional can do it at the end of the visit in a few seconds, and it costs nothing to change later. Popular slots — evenings, Saturdays, the last appointment of the day — go weeks ahead, and the client sitting in the chair is the one who gets them.
Where a studio offers a standing appointment, the same slot is reserved for you on a repeating basis and appears in your account like any other booking. Ask the studio whether they do it; it is not something you can set up yourself.
A standing appointment carries the studio's normal cancellation terms every time it comes round, so treat each one as a real booking. If your circumstances change, cancel the run rather than letting individual dates lapse — repeated late cancellations on a standing slot is how people lose it.
If you prefer not to commit, turn on the studio's reminders instead. A prompt at the right interval is most of the value with none of the obligation.