Autocomplete using API

My problem is trying to use the API SendMessage to autocomplete a cell by searching a table in another sheet.
I have 3 worksheets in a workbook. Two of them have several colomns and lots of Rows, the remaining sheet("ColourCodes") has data in columns 1 and 2 and I wish to search column one for a number typed into either of the other sheets and when found it needs to autocomplete the data from column 2 in the ColourCodes Sheet. I can do this without using an API, but the data only appears after the cell has lost focus.