how to make some entry on opened text file?

Question

hi all

I used

        System.Diagnostics.Process.Start _
        ("D:\somepath\somefile.txt")

to open a txt file.  My question is how to make some entry when the file is being opened?

say for example if I click on a buttom in my project, it opens the txt file and enter "123abc<enter>"

on the txt file