To create a dynamic text, you need to make a variable that you'll use to replace the textbox content with. In the right panel, there is a section called "Variables". Create your variable by pressing the "Plus" button.
A dialog will open where you can provide the name of the variable and a default value. Note that the default value is optional. The default value will be used if you do not provide any content for that variable when you use the template.
Now to use the variable, you can create a textbox element, select it and you will see two new fields under the variables section where you can select which variable to assign to this textbox.