Block Development: Data List in Text Field

Hi I have a select box in the form of a block I have developed.
It outputs the value chosen, on the frontend.

I would like to change the select box to a text field with a data list so that a free text value can be entered as an alternative to the predefined values, by the editor implementing the block.
Has anyone a suggestion or example of how I might do this?

Thanks,
Una

Have a look at the core select attribute. It has an option to enable a user extending the selection and hence turning a select into a combi input.
The attribute code is likely way more complicated that you need. All you will need from it is the code for the input.

Thanks for that tip John. Where do I find the core select attribute?

On github at

Equivalent in your install at /concrete/attributes/select

Thats great, thank you for your help!