SelectFilter show full name

I currently have this filter added in:

SelectFilter::make('agent_id')->label('Agent')->relationship('filamentUser', 'first_name')

SelectFilter::make('agent_id')->label('Agent')->relationship('filamentUser', 'first_name')

SelectFilter::make('agent_id')->label('Agent')->relationship('filamentUser', 'first_name')

SelectFilter::make('agent_id')->label('Agent')->relationship('filamentUser', 'first_name')

It only shows me

first_name

first_name

first_name

first_name values.

I need to show both

first_name

first_name

first_name

first_name and

last_name

last_name

last_name

last_name. How can I achieve this?