Open your design in Canva
Add or select the text box
Click the text you want to highlight
Use the text color or background color options in the top toolbar
Choose a highlight-style color for the text background if available
If needed, add a shape behind the text
Resize the shape to cover the text
Send the shape backward so the text stays on top
Adjust the shape color and transparency as needed
Group the text and shape if you want to move them together