If you're using WAB Developer edition, you can use the . replace(a, b) method in the Find widget JavaScript code (After it parses the URL params).
If, however, you don't have access to the code (arcgis.com), and are passing the data from another form, you'll probably want to encode the URI before sending, and submit the form using JavaScript. Something like:
var findParam = document.getElementById("lat-lon-input-field").value
var rootAppURL = "https://myapp.com/app"
encodedURL = encodeURI( rootAppURL + "?find=" + findParam)
window.location = encodedURL
Hopefully one of those methods answers your original question.
Ref: http://www.w3schools.com/jsref/jsref_encodeURI.asp
*There are cleaner ways to write this code, but that should get you started.