If one does not have a sense of smell:[1]

One who does not have the sense of smell [whether due to being born this way or due to a cold or stuffed nose[2]] may not say the blessing of Besamim in Havdalah even if there are other adults fulfilling their obligation with hearing his Havdalah.[3] If however there are also children who have reached the age of Chinuch fulfilling their obligation with hearing his Havdalah, then he may say the blessing on their behalf and have them smell it. [Some write that even when one does not have a sense of smell it is proper for him to try smelling the Besamim after the blessing is said by others, or on behalf of children, as perhaps his soul feels the smell.[4]]

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[1] Admur 297:7

[2] See Kaf Hachaim 297:34; Halachos Ketanos 2:183

[3] This applies even if the adults do not know how to say the blessings themselves, as all blessings of Birchas Hanehnin may only be said for another person if the person who is saying the blessing is also benefiting from it, and the blessing of Besamim on Motzei Shabbos is included within the blessings of Birchas Hanehnin.

This is thus not similar to the blessing of Hagafen said by Kiddush of which we allow one to say this blessing for another person even if he himself will not be drinking the wine, as the wine of Kiddush is not being brought because of one’s benefit from it but rather due to the institution of the Sages to say Kiddush over wine. [Hence it is similar to Birchas Hamitzvos which one can say for another person even if he himself has already done the Mitzvah.] However Besamim on Motzei Shabbos, despite it also being accustomed due to the Sages, it was only accustomed in order to benefit the person to comfort his soul which is saddened due to the leave of Shabbos, and hence its blessing is similar to any other blessing over foods and the like. [Admur ibid based on Taz 8]

Other Opinions: Rama 297:5 rules one may say the blessing of Besamim for those adults who do not know to say it themselves.

[4] Kaf Hachaim 297:36