Include subitem columns in 'Message' input field suggestions

Integration-type features are made up of Workflow blocks, which contain input fields. The ‘Message’ input field provides a text area and also displays a list of item columns, which can be added to the message.
However, this list does not include the columns from subitems.
For this reason, we request that subitem columns also appear in the suggestions of the ‘Message’ input field.
Thanks in advance.