Using a PHP function in Query Args in Builder?

Hi,

I'm trying to filter a taxonomy in builder to only display taxonomies with a certain value.

If I type it in manually, it works fine, ie

Query Args with a key of slug and a value of test, shows me the taxonomy with the slug called test.

Is there a way to run a PHP function in the value? ie, one that runs echo "test"; in this case ?

I was going to generate the PHP code and do it there, but when I press Generate PHP Code it just hangs forever. There are no errors in the php logs.

Any advice please?

Thanks
John