Export and ImportValue in another stack under sub - aws-cloudformation

Question

I am trying to get a value from one stack to another using the below syntax.
stack one-
Outputs:
CompRestAPI:
Description: Rest Api Id
Value: !Ref CompRestAPI
Export:
Name: 'CompRestAPI'
Stack two -
CompRestApiWaf:
Type: AWS::WAFv2::WebACLAssociation
DependsOn: CompApiGatewayStage
Properties:
RestApiId: !ImportValue 'CompRestAPI'
ResourceArn: !Sub 'arn:aws:apigateway:${REGION}:/${RestApiId}/${STAGENAME}-apistage'
WebACLArn: !Ref WafId
I am able to get the values for other resources using 1st syntax, but I am not able to get the value for RestApiId under !Sub
RestApiId: !ImportValue 'CompRestAPI'
ResourceArn: !Sub 'arn:aws:apigateway:${REGION}:/${RestApiId}/apistage'
So is there any way to use !ImportValue under !Sub condition?
I tried it using below code, validation is pass but still showing me an error
Error reason: The ARN isn't valid. A valid ARN begins with arn: and includes other information separated by colons or slashes., field: RESOURCE_ARN, parameter:
CompRestApiWaf:
Type: AWS::WAFv2::WebACLAssociation
DependsOn: CompApiGatewayStage
Properties:
ResourceArn: !Sub 'arn:aws:apigateway:${REGION}:/{!ImportValue CompRestAPI}/stages/apistage'
WebACLArn: !Ref WafId

Answer 1

I am done with it using Fn::join:
SourceArn:
Fn::Join:
- ""
- - 'arn:aws:execute-api:'
- !Ref AWS::Region
- ':'
- !Ref AWS::AccountId
- ':'
- !Ref ApiGatewayRestApiResource
- '/*'

Answer 2

this should work
ResourceArn: !Sub
- 'arn:aws:apigateway:${REGION}:/${CompRestAPI}/stages/apistage'
- CompRestAPI: !ImportValue CompRestAPI
you can expand the second parameter to have multiple keys for multiple imports like so
SecretString: !Sub
- 'postgres://${username}:${password}#${dbhost}:${dbport}/${dbname}'
- username: !Ref 'DBUser'
password: !Ref 'DBPassword'
dbhost: !Ref DbMasterDnsEntry
dbport: !GetAtt AuroraPgCluster.Endpoint.Port
dbname: !Ref 'DBName'