I would like to populate a custom field with information from a value related to a selection in the first custom field.

[dawnmaried10]

I have a dropdown field and I have given each choice a value (1, 2, 3). I would like to create another custom field that takes the information from the first custom field and populates it with said value.

Ex. Client Name (1st custom field with drop down) - populate Client ID # (2nd custom field) with the value related to the selection in the 1st custom field?