"As to" is a preposition like "on", "of", "about", etc.
In the expression "a preference as to ___", it means something similar to "about". It introduces the topic of the preference. Note that "about" does not fit with the word "preference".
Here are some examples:
Let me know if you have any preferences as to where to meet up.
Did Margaret express any preferences as to how we should get started?
Sentences that use "as to ___" are usually pretty formal.
Some other words or phrases that use "as to" as a preposition include:
I'm puzzled as to ___
I'm curious as to ___
Do you have any ideas as to ___