When you create extension field via Application Extension Tool, it seems the code list ( or drop down list ) is not supported by this tool. This type does not appear in supported data type list.

In fact the requirement to create an extension field with type Code List is still feasible. The entries in code list in this approach are fed by check table so we have to create a check table first.
I have the following check table:

It has a text table accordingly:

And fill some value into this check table:

Once the check table is ready, you can check extension field and assign this table to it.
The attribute "Render/Validate As" must be set as "Show checktable as DDLB":

In the runtime the extension field "JVM Type" works as below:

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