Defining a required output field parameter for an existing or a new field

Question

I need to define a required field parameter, and I'd like it to work with both existing and new fields, i.e. regardless of whether the field already exists.

The problem is, if I enter a new (non-existing field), I get error 000728: Field ... does not exist within table.

This must be due to the parameterDependencies property, but without it I would not get a dropdown allowing to select an existing field. The direction parameter does not seem to matter (Input or Output).

Basically, I'd like to be able to use the drop-down field selection when the field exists, and enter the name directly for a new field. This sounds pretty basic, I must be missing something simple. Could this be done?

Here is a sample *.pyt code to illustrate what I am trying to do.

import arcpy

class Toolbox(object):
    def __init__(self):
        self.label = "[Test] Select existing or new field"

        # List of tool classes associated with this toolbox
        self.tools = [Test_SelectExistingOrNewField]


class Test_SelectExistingOrNewField(object):
    def __init__(self):
        self.label = "Select existing or new field"
        self.description = ""

    def getParameterInfo(self):
        # Feature class where an existing field resides, or a new field will be added to
        p0 = arcpy.Parameter(
                        displayName   = "Input feature class",
                        name          = "fc",
                        datatype      = "DEFeatureClass",
                        parameterType = "Required",
                        direction     = "Input")

        # Name of the output field in the `normals` feature class which will contain the final (conflated) Z values.
        p1 = arcpy.Parameter(
                        displayName   = "Existing or new field name in the input feature class",
                        name          = "fld",
                        datatype      = "Field",
                        parameterType = "Required",
                        direction     = "Output")
        p1.parameterDependencies = [p0.name]
        p1.value = 'AField'

        return [p0, p1]

    def isLicensed(self):
        """Set whether tool is licensed to execute."""
        return True

    def updateParameters(self, parameters):
        """Modify the values and properties of parameters before internal
        validation is performed.  This method is called whenever a parameter
        has been changed."""
        return

    def updateMessages(self, parameters):
        """Modify the messages created by internal validation for each tool
        parameter.  This method is called after internal validation."""
        return

    def execute(self, parameters, messages):
        pass

Answer 1

Similar response to Midavalo on the 13th May except add a Boolean parameter so the user can select an existing field or enter a new field. i.e. Add two additional input parameters

1. Boolean parameter to process an existing field from an input feature class or a new field
2. New field