Because currently it's only possible to add 1 custom search engine in WebPositive I've been looking for solution using external script.
First I created a launcher that I can make executable and double click from Tracker:

#!/bin/bash
Terminal python3 "/boot/home/Desktop/Web+ prefix/launcher_py"

I don't know how to eliminate that fixed path here, I'm not an experienced bash user, ideally it would be relative to the script folder.
So here is the second script:

x="\""
import os,sys
fold = os.path.split(os.path.realpath(sys.argv[0]))[0] #working directory folder
a=os.listdir(os.path.join(fold,"pages"))
li=""
for i in a:
	li+=x+i+x+" "
b=os.popen("hdialog --radio \"What website urL you want to prefix?\" "+li).read()
os.system("Terminal python3 "+x+os.path.join(fold,"pages",b[:-1])+x)

As you can see it's scanning the directory "pages" residing in script's directory, for definitions of search engines. The user interface is made using hdialog application from HaikuDepot.
The user input and call to system to launch the browser is made in the search engine definintion residing in pages sub-directory:

import os
print("Paste word to check it in wiktionary:")
url=input()
os.system("WebPositive https://wiktionary.org/wiki/"+url)