Sorry, I miss understood your question, please review following code written for txt file, however you can change it for Json file. attached snapshot shows two parameters, Folder and List of Strings. Once you select the folder containing text file, using parameter "Add dialog box", your second parameter will be populated with text list.
def updateParameters(self):
user_path=self.params[0].value
data =os.path.join(str(user_path),r'Msb.txt')
results = []
with open(data) as data_file:
for line in data_file:
results.append(line)
self.params[1].filter.list = results
return