How to add content (text) to an ASCII based file

[kent@koylia.org]

I write 98% of my research oriented macros for the academic research suite Nota Bene. The data is stored in ASCII.
I need to add simple content to these files such as:

Find "~ct#"

THEN go to the end of that line and add a time stamp from the clipboard, such as "cit#-1506061411111"

HERE IS WHAT I WOULD LIKE TO DO:

I want this to happen 'behind the scenes' and
Open this as an ASCII document in a hidden state (or minimized)
Find the "~ct#" (I think I NOW know how to do this)
paste the content from the clipboard at the end of that line.

SO HERE IS MY QUESTION:

What is the best way to do that? Or is there some way to treat an ASCII file that is hidden?

I can do it now by:
-opening as text with Notepad
-find the text via 'windowtext.find'
-keyboard to the end of the line
-paste from the clipboard

BUT I am just wondering IF there is a more efficient way to do this?