(This answer uses American English)
As a note, the common meaning of "appointment" is "a scheduled time to meet with someone in a professional context". You would say "I have a doctor's appointment today" or "I have an appointment to see Mr. Smith at 2pm."
In the context of official government positions or high academia, it is correct to say that someone has been "appointed" to a position, and if you stretch the grammar you could say they have had an "appointment", but this usage is extremely uncommon. Even in this context you would say "they were appointed" instead of "had an appointment". "Appointment" is almost exclusively used for a single scheduled meeting.
Within a typical business, "appointed" is not the correct terminology for being assigned a job, role, or duty. The term "appointed" is a much higher register, that is, much more formal than for normal business activities. It really is only government and high academia that use "appointed".
Now, you are looking for a certificate that confirms that you were employed to a specific position within a company. In America, this is an uncommon request*, and as such, it would be useful to simply describe the object you need as I did here: "a certificate that confirms that you were employed as a {JOB TITLE}."
However, you could give this certificate a title. Many constructions would be acceptable, as there isn't really a standard term. Examples could include "Employment Verification Certificate/Letter/Notice", "Proof of Employment", and so forth. Again, note that "appointment" would be unlikely to be used unless the position was with a government agency or university.
* - In America, it would be more common for your new employer to call your old employer and simply ask them to confirm you worked for them.
