Version: 18.104.22.168 – Build ID: 22.214.171.124-1.fc28
CPU threads: 4; OS: Linux 4.18; UI render: default; VCL: gtk3;
Locale: en-GB (en_GB.UTF-8); Calc: group
Features request: support in Writer for Formatting mark and character-space.
Current state: tool Find & replace (Ctrl + H) | Other options –with defaults settings– | Formats...., does not provide support for operation formatting mark (Ctrl + F10) as Find object and character-space as Replace object.
Perhaps the following extension is able to solve your problem (but I'm not sure)?
Created attachment 144947 [details]
AltSearch – shortcut creation instruction
Latest release installed successfully (v.1.4.2, released Mar 24, 2017). Nevertheless invoking keys Ctrl + H calls still the built-in Find & replace tool instead of the installed one.
The current instruction do not apply to Libreoffice:
*** This bug has been marked as a duplicate of bug 38261 ***