Hello, Jorge Martin!
Unfortunately, seems that no way to achieve that using the Parallax background shortcode widget.
Instead, you can have the same look using the Text Block widget with the parallax background applied to the row.
Please check the video guide I made for you:
Parallax H1
After you created the content using the video guide, if you want to change the width of the row and font size for your text, insert the following code into Customizing->General->Additional CSS:
.parallax_h1 h1{
font-size:2vw;
}
.parallax_h1 {
min-height: 300px!important;
}
Change the Font-size and min-height values per your needs.
Regards