How to reference userpool arn from function

Serverless Framework
1

I am trying to access my serverless created cognito userpool in a function declaration. However the usual Fn::GetAtt: does not seem to work. What can I do to get the ARN of the user pool to use as the authroizer.

Example yaml:

functions:
	graphQL:
		handler: graphql.handler
		events:
		- http:
			path: graphql
			method: post
			cors: true
			authorizer:
				name: authorizer
				arn:  #not working#
				  Fn::GetAtt:
					- UserPool
					- Arn
					
resources:
  Resources:
      UserPool:
		  Type: AWS::Cognito::UserPool
		  Properties: 
			DeviceConfiguration:
			  ChallengeRequiredOnNewDevice: true
			  DeviceOnlyRememberedOnUserPrompt: true
			AdminCreateUserConfig: 
			  AllowAdminCreateUserOnly: false
			AliasAttributes: 
			  - email
			AutoVerifiedAttributes: 
			  - email
			EmailConfiguration: 
			  EmailSendingAccount: DEVELOPER
			  ReplyToEmailAddress: contact@[redacted]
			  SourceArn: [redacted]
			LambdaConfig: 
			  CustomMessage: 
				Fn::GetAtt:
				  - CustomMessageLambdaFunction 
				  - Arn
			  PreSignUp: 
				Fn::GetAtt: 
				  - PreSignupLambdaFunction
				  - Arn
			MfaConfiguration: OFF
			Policies:
			  PasswordPolicy: 
				MinimumLength: 8
				RequireLowercase: true
				RequireNumbers: true
				RequireSymbols: false
				RequireUppercase: true
				TemporaryPasswordValidityDays: 7
			Schema:
			  - AttributeDataType: String
				DeveloperOnlyAttribute: false
				Mutable: false
				Name: email
				Required: true
			UserPoolName: ProjectR
			UserPoolTags: 
			  project: "Project-R"
			  state: "Project-R-Dev"