How to filter values in lookup based on URL string

I have a CRM portal form that has a lookup field displaying as  drop down values. These drop down values are coming from a custom entity. we have close to 1500 drop down values on the form. I would like to show values based on the URL string. Ex. If url string contains PK then I would like to show only values from the custom entity that has code PK. In this case my custom entity has 261 values that have code PK. Similarly if the URL string contains MS then I want to filter the 1500 drop down values into 275 as my custom entity has 275 records with MS as the code. I have a custom web template to render the form in portal. What is the simplest way to achieve this. I am open to add js or liquid. 

Thanks for suggestions

R89