Depending on trigger and condition (e.g. when issue created and for bugs only):

1. Trigger: When issue created.

2. Condition: Issuetype equals the needed one.

3. Then: Edit Checklist Text field

   1. Here we paste the text like appears in the checklist editor or reuse current content of the field, e.g.
      {{issue.customfield_1234}}

   2. Then, we add a line (the task we need to assign with the custom field). We use smartvalue in this way:
      @{{issue.customfield_5678}}