Hi partner,
According to your description, did you mean you want to use JS to change the value of lookup field and trigger the onchange event?
If so, the setting value event is triggered when loading the form, not onchange event and therefore you could not trigger the action of change event.
I recommend to try "Quick View" OOB feature.
Go to form editor and switch to "Insert" tab->Quick View Form


Here is Microsoft description of "Quick View".
Hope it helps.
Best Regards
Leo