You can't. You'd need to manually drag fields in this dialog to align the right fields with the columns of data in your tab file or edit the data in the tab file to strip out the spaces.
BTW, this won't "create" a field like you describe, the data in the field simply won't get imported instead of being imported into the correct field.
Ok thanks for the confirmation. Interesting to note: when importing from a text file into a new table or DB. the tab delimited text header "Sequence " will cause FM to create a field named "Sequence" without the trailing space. Which it then wont match on subsequent imports.
Won't match on subsequent imports where the column name is "sequence "? That would be expected behavior as far as the field name matching goes and trailing spaces could cause problems with FileMaker names, so I'd prefer to see them trimmed like this anyway.