Is it this you are looking for?
DialogInput[{filename = FileNameJoin[{$HomeDirectory, "Desktop"}]},
Column[{"Either type your file name (in quotation marks and with the \
file extension) or browse for your file:",
InputField[Dynamic[filename], String,
FieldHint -> "Enter your file name"],
FileNameSetter[Dynamic[filename],
"Open", {"text files" -> {"*.txt"}}, Method -> "Preemptive"],
DefaultButton[DialogReturn[filename]]}]]
Henrik