Monkey Boy
if it is always 10 digits, you could try

Sub ChangeFormat()
Columns("A:A").NumberFormat = "000000-0000"
End Sub

"spreadsheet monkey" wrote:

I need a solution/macro to insert a minussign (-) into a 10 digit number
between digit 6 and 7 (to get a number looking like this xxxxxx-xxxx)

I need to do it in a single column for all 65 536 lines of code in the
spreadsheet.

who of you is nerd enough to tackle this challenge?