Combining Fields from user input content fragment in grapgql

There are 2 tag fields in my content fragment model lets assume the names as tag1 and tag2. User creates a content fragment and selects the desired tag values. But I want to take these 2 values and combine them with an underscore. is there any command that permits this in graphql or is there any other solution to this. can I make a separate field for it in crx/de and then obtain it’s value in it. if yes then what should be the parameters for this or should use some javascript in it. please help.

I tried to concat the fields in graphql only but it is not working
desired output:- tag1_tag2 :— want this value for filtering