Two possibilities for the object of representation 于 are:
Pictogram (象形). An ancient wind instrument; early form of 竽 (OC *ɢʷa). This view is supported by mainstream palaeography.
Exhaled air; early form of 吁 (OC *qʰʷa, *ɢʷas, “to sigh”). Shuowen considers it to be an ideogrammic compound (會意): 一 (“flat stream of air”) + 丂 (“to exhale”). Compare 乎, 兮, 号.
I tried to google 竽, but could not find the ancient wind instrument. could you please link to this instrument they are referring to? In this case how is the meaning transformed to “in, on, at, go to”?
I was searching for 由于 yóuyú 'due to'. 由 'reason' + 于 'go to'?