How can I prompt for variables when launching Azure DevOps pipelines?

I'm trying to get Azure DevOps pipelines to prompt for a version number when manually launching a pipeline (defined using the new YAML syntax).

Even when I define variables in the template, the launch screen says "This pipeline has no defined variables". How can I define variables so that they show up in the pipeline launch?

Current YAML definition contains:

variables:
  - name: versionName
    value: ''

These are not shown when launching the pipeline:

While Shayki's answer is correct for defining variables, what I was really looking for is runtime parameters.

With the following YAML definition:

parameters:
  - name: myParameter
    displayName: Description of myParameter
    default: defaultMyParameter
    type: string

it prompts for the parameter value when launching the pipeline:

The parameter must be referenced in the template using ${{ parameters.myParameter }}, the other variable syntaxes don't work.