There is no such button in Spatial References dialog like ArcMap, it seems that ArcGIS Pro doesn't have this function
Well there is the Spatial Reference class in arcpy/python or you could edit a prj if the changes are minor, and there is a tool for use in modelbuilder of course, you could just use a projection file created in 10.4 in pro
Did you check Create Spatial Reference—Data Management toolbox | ArcGIS Desktop ?
Jayanta... that is my highlighted modelbuilder link... it only works for it
Oops!!! My mistake. I should have checked the links.
It is planned for an upcoming release! Generally, we're not supposed to promise which release will have new functionality, until very close to the end, but we are close to the end...you should be able to create custom coordinate reference systems in 2.0.
Retrieving data ...