You can use Arcade expressions to do this. In ArcGIS Online, select the layer > Configure Popup and below the Pop-up Content area add a new Attribute Expression.
var orig = "http://www.dot.ca.gov/hq/env/gis/stormwater/sdsi/d7/photos/dp/110_s_22_12966_dis_34_0358_118_2749/index.html" var origSplit = Split(orig, "index.html",1) var id = $feature.objectid var new = Concatenate([origSplit,id,".jpg"]) return new
Select Test and look at the returned result. Select OK and then in the Pop-Up Content, you have the option to show the results of the query in the popup window. This solution means that you are doing all the calculations dynamically which is nice because then you don't have to edit data from the original layer.