AccessForm:AfterUpdateEvent code writing

FieldName “MainDTCode” is set for DATA TYPE=text and limits the input range (1-999). I am currently utilizing 1-165 of the range.
Each one the those entries requires a corresponding “Code,” limited to only: MTN,PRO,OTH,SET and RAW.
How do I write the code so that when the user iputs a number into the Form\FieldName\ “MainDTCode”, it auto fills the FieldName\”Code” with one of the five alpha codes?
e.g.
If MainDTCode=”54″ then Code=”CHG”
End If
Is this correct? Please be specific.