simple: You can't
It's not possible to escape an equal sign in the search string.
There exist some pure batch solutions (also on Dostips), but I would recommend the easy to use and powerful solution from Dave JREPL
This is a JScript/Batch hybrid from one of our honored, expert members.
If you still insist on using a pure batch solution, you can search this forum (probably tougher than writing your own solution).
One pseudo code solution could be:
Build a modified_string
1. Replace all hash signs with #H in your string
2. Replace all equal signs with #E in your string
Obviously the second point is problematic, too, but can be solved by
2b. Loop over all characters of the string and check if it is an equal sign
3. Do the same for your search string
4. Execute the replace expression result=!modified_string:modified_search=repalce!
5. Replace remaining #E back to equal signs by: result=!result:#E==!
6. Replace remaining #H back to equal signs by: result=!result:#H=#!