In some situations you might want to prefill some fields for the user. This can be achieved by the following method. 
Gleap.preFillForm({
    description: "This text will be prefilled.",
    errordescription: "This text will be prefilled as well.",
    someformidentifier: "This text will also be prefilled."
});
You need to pass a key value pair to the preFillMethod. The key is the form identifier, which can be found under the ‘Advanced’ section of a question. (see screenshot for an example) Form identifier