Details TBD.
Draft steps:
Trigger: When issue created/transitioned/updated
Condition: e.g., issue type equals the needed one.
Then: Edit Checklist Text field
Here we paste the text as it appears in the checklist editor or we reuse the current content of the Checklist Text custom field, e.g. {{issue.customfield_1234}}
Then, we add a line (the task we need to assign with the custom field). We use smart value in this way:@{{issue.customfield_5678}}