I am wanting to constrain a centerline in Model Space using an edge of an existing feature. For example in Solidworks I would just RMB the edge and select "Select midpoint" from the dropdown and then ctrl+click the line and make them coincident.
Referencing edges is less stable than referencing surfaces, so you should find a way to do that if you can. If your reference is a cylinder, for example, you could create an axis through that cylinder and reference the axis.
If you really do want to reference the midpoint of an edge, I believe you'll have to use a point in the sketch. Create the point (anywhere you like!), then create a Midpoint constraint between the point and the reference edge; and then you can draw a centreline through the point.
Create a sketch with a point, or 2 points. Constrain the points. Now create the axis to the point and a perpendicular plane... or the 2 points of the sketch. Point being <pun alert> you now have a constrained feature to tie the axis too.
You can also extrude a hole in space but this always gives a design intent error.
I believe you can use a relations. All depending from what you what to see in the result.
Example, the axis in the mid of an edge: d5=d2/2
Thank you everyone for the suggestions. What I ended up doing is creating references on the edges I wanted to select and then creating a construction line that would start and end at the references just created, after that I just set a point and constrained it using the midpoint, and finally I added a centerline normal to the construction line and coincident to the point created.