Just start entering numbers in that box. The program will put a dash after the first 2 numbers and will not allow you to enter any more numbers after you type in 9 numbers.
Don't worry about putting a "fake" number in there. ABSOLUTELY NOTHING contained in the 1099-MISC is actually transmitted to the IRS except for the dollar amount. All the IRS sees is a lump sum number on line 1 of Schedule C - "Gross receipts or sales" - which reflects all revenue you are reporting from any and all sources.
Tom Young