Form I-94 is the DHS Arrival/Departure Record that denotes the date of admission to the US for foreign citizens who come to the US on a non-immigrant visa (i.e., a work visa such as H1B). The form is electronically generated for arrivals by sea or air; a paper form is still provided at entry by land.
Those with work visas can enter their Form I-94 number and its expiration date (if any) during onboarding into Zenefits. This information will automatically populate in Section 1 of the I-9 in Zenefits.
Travelers who do not know their I-94 number can retrieve it from the US Customs and Border Control site: https://i94.cbp.dhs.gov/I94/request.html.