href is a string that can contain an URL address wit some parameters in the format of query string...
par1=val1&par2=val2
If you have the values you can use
jQuery's param[
^] method to create such string from a JavaScript object easily...
var params = {
"param1": GhestCompanyID,
"param2": ExternalGhestID,
"param3": ExternalCellPhone
}
document.location.href = "frmInstallmentPayment.aspx?" + $.param( params );