Adding a text to a cell in Excel using Formula requires use of ampersand operator, CONCAT function, or the LEFT, RIGHT, and LEN functions.

Click the "Insert" tab and then click the "Text Box" button in the ribbon's Text group. The cursor changes to a "T." Drag the Text Box Tool next to the Gantt chart to create a text box.

In the "Address" text box within the Insert Hyperlink dialog, type "mailto:" (without quotation marks) followed by the email address you want to link the first hyperlink to.