Web form field – Select box with free text

I've been tasked with building a component to allow the selection of some sane defaults or optionally allow the user to enter free text.

Semantic UI offers an Allow Additions option which enables the user to enter free text on a select drop down.

Semantic UI: Sane Defaults Semantic UI: Sane Defaults

Semantic UI: Optional Free Text on Dropdown Semantic UI: Optional Free Text

Is this optimal for usability and if so why?

My initial thoughts are this is too similar to a searchable dropdown. Would having a second input field help; Where the user explicitly selects Other in the dropdown and then a new input field is displayed. I don't know if I expect this behaviour from being trained by other forms. Is it bad usability to display new input based off current select?

Semantic UI: Searchable dropdown Semantic UI: Searchable dropdown

Custom Input on selection of Other Custom Input on selection of Other

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