For approved WhatsApp templates, the message body populates automatically and is not editable. The variables in the body are mapped to the columns in the contact file after the file has been imported.

The column headings in your file are automatically shown as options in a drop-down for each variable in your template. From each drop-down, select the column that must be used to populate each variable.

Merge fields are currently not supported for SMS – this is on the roadmap.