Linking to a text box in a sheet is straightforward, unless you want to copy that linked text box to another sheet and retain the link. Here is how you do it.
To link to a text box you click the text box and then click in the Formula Bar and press = and then click the cell to link to and press Enter.
This works OK on the sheet but if you copy the text box to another sheet it links to the same cell in the other sheet. If that’s what you want, great. If it isn’t then you need to use this technique.
Text box copy technique
Click the text box click in the Formula Bar and press = then instead of clicking on the current sheet click on another sheet tab and click a cell in another sheet then return to the current sheet and then click the cell you actually want to link to and press Enter.
By doing it this way the sheet name is included in the link and that ensures the link is kept when you copy the text box to another sheet.
You could also manually type the sheet name into the Formula Bar, but using the mouse is much easier.