To modify a layer in Simple URLs, you'll need to define your layers in sets of 3, using the following format: [ layer/attribute/data ]. Repeat this format for any additional layers in your template that you want to modify.
In the example below, "message" is the layer name, "text" is the attribute, and "Hello World" is the data input for that layer of text. Take a look at the result.
Your Simple URL base will provide a placeholder text with all the layers in your template that you can modify and a list of the available modifications as shown below.