Adding the button was so straightforward.
I decided to add Find * IN {sourcepath} CONTAINING {dlgstring|Enter search string} to the right hand side of the pair, and it was also added to the left hand side of the pair. There is a question brewing about whether this is the bext way to do it, or is it best to have a single button on the toolbar above and select which pane. Presumably that's just a choice?
Also, when using the search, if a single word is entered into the dialogue box like cat it works perfectly but if cat dog is entered the Find Files window is shown with whatever contents there were from the last time used. Presumably Find doesn't know what to do with the new dog sub-command.
The source is here Quickly search in files - #2 by Leo and I think I need to modify it in some way to keep the multi-word search string intact.
I have a workaround which is just to put "cat dog" with the quotes into the search box but it would be nice to learn how to treat it as a single token within the Find internal command.
As a new user of Obsidian and the use of markdown files it would be great to be able to search the text files, and get even greater use out of dopus.