Carlos:
You can use a function called QueryString(). Here is the description from the GridView help file.

"This function prompts the user to enter a string, the default being blank or the Optional String if present. In all cases, the last string entered becomes the new default string for subsequent refreshes.

Left({Telephone},3) =Left(QueryString(”Enter Area Code“, “604”),3)

When the filter is enabled, the function prompts the user for the area code and then returns all records where the first three digits of the phone number match the area code entered. The value string defaults to “604” in this example."