Cognito custom authentication flow - initiateAuth giving error

I am trying to make a custom authentication flow using AWS Cognito so that i can send MFA codes via email instead through the cognito triggers. I am using the initiateAuth() method to do this which is correct according to the documentation;

https://docs.aws.amazon.com/cognito-user-identity-pools/latest/APIReference/API_InitiateAuth.html https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/CognitoIdentityServiceProvider.html#initiateAuth-property

My payload seems to be valid but when i try login with a user i get the error 't.getauthparameters is not a function'

I've had a look through some other stackoverflow posts but nothing is helping

Any ideas what is going wrong?

This is a snippet from my code below:

const payload = {
		  AuthFlow: 'CUSTOM_AUTH',
		  ClientId: 'my client id', 
		  AuthParameters: {
			 USERNAME: $('input[name=username]').val(),
			 PASSWORD: $('input[name=password]').val(),
			 CHALLENGE_NAME: 'SRP_A'
		  }
		};
		
		cognitoUser.initiateAuth(payload, {
			onSuccess: function(result) {
				// User authentication was successful
			},
			onFailure: function(err) {
				// User authentication was not successful
			},
			customChallenge: function(challengeParameters) {
				// User authentication depends on challenge response
				var verificationCode = prompt('Please input OTP code' ,'');
				cognitoUser.sendCustomChallengeAnswer(verificationCode, this);
			},
        });