Assigning a name to Deal Name from a drop down list in Deals Module

Assigning a name to Deal Name from a drop down list in Deals Module

I simply want to auto assign a value to deal name from a drop down list box field which is selected from within the deals module. This value will be text which contains both letters and numbers.

Instead of typing a deal name, I'd like to select it from a drop down list which is a separate field I have created within the deals Module.

When a deal is saved, I'd like this deal name to be updated from this field drop down list?

Any solutions......... thanks