Hello,

You cannot add an Instagram share button because Instagram does not have a share option.

It has only to follow options you can set the link in the Theme Settings >> Social profiles >> Links to Social profiles.

Screenshot for Clarification: https://ibb.co/m87kRTd

Then you can display it in the header by editing the social icons buttons and set them to follow.

Screenshot for Clarification: https://ibb.co/zXwwRmd

Regard’s.
Xtemos Studio.