That will require some CSS.
First edit the Headline block, in Spacing set the Border Bottom width you require, then set the Colors > Border. Then in Advanced > Additional CSS Class(es) add: dotted-border
.
Publish those changes.
Then add this CSS to your site.
.dotted-border {
border-style: dotted;
}
This post here explains the various border-style properties:
https://www.w3schools.com/cssref/pr_border-style.asp