How to add story point field

To add any field to the Scheduler wizard, all you need to do is add this field to the 'create issue screen':
https://support.atlassian.com/jira-cloud-administration/docs/add-a-custom-field-to-a-screen/

The easiest way to check, where field should be added is to use the feature 'where is my field' on issue creation (project and issue type are important), remember this feature is only visible for Jira admins.

In case of the story point it might be tricky, because in Jira Cloud there are team managed and company managed projects. Please check this question in community to learn about the differences:
Story point/Story point estimate fields

Apr 12, 2022 New article on community about adding story points: How to make “Story points” appear on issues?