If the article that you are referring to is in a journal supplement you would provide this information in place of the issue information (in round brackets). In addition to this, you would add an 's' before the first page number.
This is how an article from a journal supplement would appear in your reference list:
Tang, R. and John, S. (1999) The 'I' in identity: exploring writer identity in student academic writing through the first person pronoun. English for Specific Purposes, 18(suppl. 1) s23-39. Available from https://doi.org/10.1016/S0889-4906(99)00009-5 [accessed 3 July 2018].
or:
Tang, R. and John, S. (1999) The 'I' in identity: exploring writer identity in student academic writing through the first person pronoun. English for Specific Purposes, 18(supplement 1) s23-39. Available from https://doi.org/10.1016/S0889-4906(99)00009-5 [accessed 3 July 2018].
