I always thought it was 'für' but then I saw 'fürs' in the sentence "Danke fürs lesen"
fürs=für + das( for + the ). danke fürs lesen-thanks for the reading.
Can you just say 'für'?
Not in that sentence, no.
German uses a definite noun there - das Lesen, the (act of) reading, the specific act of reading that just happened and that you are thanking them for.
https://en.m.wiktionary.org/wiki/für#/search. there are some examples of use on this wikipedia page.
'fürs' is short for 'für das'