One way is to select a joint from the starting templates or from the Connection Library database in the Design tab and modify it according to your needs.
If you do not find a similar template, you can use several operations - Widener, Rib, Stiffener plate, or even Stiffener member.
While the Widener operation allows you to model the haunch with flanges quickly, it also cuts the visible member length. If you want to model the full member length, you need to use the other operations and assemble the haunch.
Using the operation Stiffening plate, you can even import the shape of the plate from a DXF file. All three above-mentioned plate operations can then be edited via Editor - chamfer a corner, insert an opening, etc.
In the example shown during one of our webinars, you can learn how to design a haunch of an IPE cross-section member.
Another example was shown in this webinar.