I want to add some details into a new lead form on the click on a button.

I have written the code to connect to an external api using getUrl and can get the address information i need from it into a variable but I can't find how I can put information from this variable into the text boxes on the form. My plan is for my users to complete the other data that is not imported from the API and then save this as a new lead.

Is it possible to update the new lead form in this way or should I be trying to do this in a different way?