Your paper, without any of the supplementary material, should be clear and not easily misinterpreted, even if it must be superficial due to word limits. It should also be clear as someone reads along when they can view supplementary material to find more depth if they are interested. You should not need to inform the reader separately that they should read the supplementary material, your references to it should be sufficient on their own.
An exception may be journals that move substantial entire parts of the paper to supplementary material, like all or most of the methods section. In that case, usually the journal has its own way to direct the reader to "supplementary methods section" or "online methods" or similar; look at other papers published in the same journal for guidance.