Horsham face a trip to Hayes & Yeading in the third qualifying round of the FA Trophy.
The Hornets were drawn against the Conference South side at Wembley Stadium this morning.
United are managed by former Liverpool defender Phil Babb and currently play their home games at Maidenhead United.
The final qualify round will take place on Saturday, November 29, with a place in the first round proper and £4,000 up for grabs.
Horsham made sure they were in the hat with a 3-1 win away at Ryman League North Division side Barkingside on Saturday.
Having trailed 1-0 after nine minutes, a brace from Jamie Ampleford and Tony Nwachukwu goal sent them through.