Add custom field to a screen

Open an issue page (make sure it is an Old Issue view)
?oldIssueView=true URL suffix forces Jira to show the issue in Old view, e.g., https://gebsun-support.atlassian.net/browse/IC-159?oldIssueView=true
Expand the Admin dropdown and click Add field option as presented below
The dropdown with list of available fields pops up. Select the field that you want to add to the Issue View, e.g., Checklist Content YAML or Checklist Text:

Go to the selected project page and select Project Settings option in the sidebar navigation:
Next, select Screens in sidebar navigation:
On the Screens page, click a name of a screen that you want to configure (depending on the operation and issue type), e.g. a screen for Create issue or View issue operation:

You can also see the list of all screens available in Jira by going to Jira Settings > Issues > Screens page.
On the Configure Screen page add a field (e.g., Checklist Text) to the list:
Test your setup: go to the Create Issue (or View Issue) page and spot the custom field available, e.g.:

Issue Checklist Documentation (Legacy)