How to Concatenate Two Values into FormComboBox textField

I'm trying to concatenate two values into the textField property for FormComboBox, but it doesn't allow me. Instead I get an error:

Uncaught runtime errors:
ERROR
_utils__WEBPACK_IMPORTED_MODULE_2__.getItemValue(...) is undefined

My code is below. Perhaps someone can help?

<Field
	key={'personU'}
	id={'personU'}
	name={'personU'}
	label={'U'}
	textField={'abc' + 'def'}
	dataItemKey={'uic'}
	placeholder={'U...'}
	component={FormComboBox}
	allowCustom={true}
	data={units}
	virtual={{
		total: uCount,
		pageSize: uPageSize,
		skip: page
	}}
	onPageChange={pageChange}
	/>

 I've tried the following:

textField={'abc' + 'def'}

textField={`${abc} ${def}`}