Help with customising existing dropdown menu

Hi everyone,

I am trying to customise the dropdown menu options on the map search feature.

I would like to add additional options to extend the range.

Currently you can only search the map within a radius of 100 (km or miles) - I would like to be able to search 500, 1000, 5000, 10000.

Can anyone please help me with some script that I can add in to customise that search dropdown menu. It is not a dropdown from a list I wrote, but the one that comes with Knack.

Any suggestions gratefully received!

Thank you!!