How to find out my plan to create a web app in Azure

I'm trying to create a webapp as suggested here. The group is created. Then, I omit the step of creating a plan, since I already have a subscription allowing me to do that (which I can confirm by creating the app from the GUI and picking which plan/subscription I want to use).

When I execute

az webapp create --resource-group my-group `
                 --name app-lication `
                 --plan "Enterprise MSDN Dev Test"

it fails, since the plan isn't recognized. I tried with different names and GUIDS (those I see when I az login, for instance etc.) but nothing seems to work out. How do I obtain the correct referece to my plans/subscriptions so I can pass it as the last parameter in the command above?

It's going to be more reliable to pass the Plan ResourceId than the name, this specifically caters for scenarios when the plan exists in a different resource group.

The naming convention of your plan in the question is typical of an Azure Subscription. Ensure you do have a properly created App Service Plan to use.

AppServicePlanID=$(az appservice plan show -n SharedAppServicePlan -g MyASPRG --query "id" --out tsv) 
az webapp create -g MyResourceGroup -p "$AppServicePlanID" -n MyUniqueAppName

https://learn.microsoft.com/en-us/cli/azure/webapp?view=azure-cli-latest#az-webapp-create-examples

As mentioned in comments, an App Service Plan is different from a Subscription.

An App Service Plan is a kind of resource which will host your web apps. Think of it as a VM where you can host multiple web apps.

What you have to do is first create a new App Service Plan using az appservice plan create and use its name/id when creating web app.

You can learn more about it here: https://learn.microsoft.com/en-us/azure/app-service/overview-hosting-plans.