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I'm not support, but you could use logic conditions.
In a nutshell, setup dropdown last option as "others" for example. Make condition logic so that when "others" is selected, a hidden field will be visible. The hidden field can be a text field.
It's true you guys aren't part of our support team but they're both good answers to bacccdc's question :)
If you wish further assistance with this or anything else JotForm related, we'd be happy to provide it.