I've copied below an exchange we had in the group a while back  that
should get you started on this custom function.

To pass a cell  value to VBA and back is quite straightforward, but made
immensely easier if you give the cell a range name first. For instance,
on a sheet  called Front sheet, the cell at A5 is given the range name
This_One. Its value can be passed to a VBA variable  valled varValue with
this line:

varValue=Sheets("Front sheet").Range("This_One").Value

You can write it back to the cell like this:

Sheets("Front sheet").Range("This_One").Value=varValue

I've been meaning to find out if this worked. Have named
cells all along, but not since getting into VBA SUPER! Now I can
move things around and not have to parce my code for the references.

FYI, Abhishek,
You reference cells like this (absolute) too (some code snippets):

Get from the sheet  to VBA
f = ActiveSheet.Cells(Row, Col).value ' (.value is optional)
PortName = ActiveSheet.Cells(1, 12).Value

Going the other way
ActiveSheet.Cells(r, 2) = ModeName