datagridview change value in cell in editmode programmatically

Version : C# 2008 express

I have a datagridview with one row.
When I am in cell 3 I enter editmode and type something, eg 6
Now when I press F2 of F5 a new window has to open with the text from the cell as parameter
First I could also not get the keyevents from the datagridview, so I set formpreview to true and used the keyevents from the form instead, that part also does work.

In that form I get a list of id numbers starting with that 6 (but its all string values)
If I click a value of that list the form has to close (dialogresultok) and passing the value to the parent form (e.g. 60003)
Now I want to have that value entered into the cell I pressed F2/F5 in.
And thats the part that does not work.
Also tried with EditedFormattedValue, but that seems to be read-only

As nobody knows what you have coded you will need to do a short demoproject zip it and post it so we can look at what you have done and whats needed to do to get it working. a description alone per sure is to less to help you in your case.