Autopoulate Field Based on Lookup Field

I created a custom module:

Module

CustomModule5

API Name

CustomModule5

Displayed as

SIC Codes (Plural Form)

SIC Code (Singular Form)

It contains 2 fields that I care about:

SIC Code	Single Line

SIC Name	Single Line

I'm trying to set this up in the Leads module so that I have a Lookup field for the SIC Code that is pulled from the above and then a second field (SIC Description) is auto-populated based on the corresponding descruption from the SIC Codes module.

I know I need a custom function but I haven't been able to get it right.

Does anyone have some code they throw my way?

Thanks.

Also, I found this:

https://help.zoho.com/portal/en/community/topic/how-to-auto-populate-based-on-previous-field-entry-from-linked-lookup

I'm guessing I could adapt that somehow?