Please remember that when you are filtering you will need to use capitals, as filtering is case sensitive.
To turn on the filter click the 'Filter A/C No.' Button as shown below:
Every time that you enable the filter, it will automatically filter the A/C you selected first when opening the contact manager. In this case it reverts to the original TURB01 A/C Code:
You will see that the filter is on by the indicators below:
Once the filter is on, you can delete the A/C, enter text in a different field (remembering to capitalise the text) and press the enter/return key to filter by that value instead:
Then type in the filter you wish and press the enter/return key. The image below shows the filter applied after the key has been pressed: