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.:

Add custom field to a screen