If you have an existing Employment Authorization Card (work permit), you can find your A-Number there. However, many people filing Form I-765 may not have an A-Number. Only use your own A-Number; using a false A-Number will result in your application being rejected.
In the example below, the A-Number is listed as “000-000-773” underneath USCIS#.
If you do not have an A-Number, provide your I-94 Number from your Arrival-Departure Record.