Sumatra PDF is a PDF, ePub, MOBI, CHM, XPS, DjVu, CBZ, CBR reader for Windows

Inverse search command via DDE

Is there a way to set the inverse search command line (InverseSearchCmdLine) by using a DDE command?

There is little need (I dont think its practical, even if it were possible)

DDE commands are incoming repetitive control

The inverse search command line is only needing to be set once per user so no problem to hand write it once or include it in a first time installation startup command line.

-inverse-search should NOT generally vary from day to day using it repeatedly or changing it , risks corrupting settings much the same as constantly overwriting a registry value (why do that?)

SumatraPDF will attempt to set it automatically if it can from existing registry values, so it can some times be auto triggered and populated by an external DDE call from one of a few historic calls. Fallback is to set it to use notepad.exe !

Only one TeX editor still insists on DDE inputs. The very popular LyX uses Memory Pipes which are similar. Java Editors may use Memory mapping via other mechanisms like web ports.

Almost all others of several dozens of editors set -inverse-search via CLI not DDE

So in direct answer to your question it is potentially possible
using a poor DDE command session may somehow trigger an inactive installation
into setting notepad.exe as the inverse search command line
however I suspect that is only of value for testing not real world use.