Thanks Pete for pointing me at those directions.
Mouse macros function didn't work with this panel.
I looked at JS41 LUA script made by guenseli user in 2011, but it didn't work, however I managed to identify some working parts and add infinite loop for continual engagement. I use "Lua Kill" control to terminate that loop when I no longer need aircraft to follow track to GTN. In the end it works perfectly now.
Guide to couple JS41's autopilot and Flight1 GTN 750 (for those unfamiliar with LUA scripts / searching for ways to couple them).
1. Create a LUA file in Modules folder, e.g. PMDG_JS41_BTN.LUA
2. Add the following lines in it:
while 1 == 1 do
3. Open FSUIPC4.ini and add under [LuaFiles] section (add this section if not present)
4. Go to Sound\PMDG\J41 and rename or delete pmdgj41_rmu_knob.wav (since LUA script calls NAV button several times per second, you don't want to hear constant clicks).
4. In Fsuipc, go to Key Presses tab, on the left press "SET", press some keyboard button, in "Controls sent keys pressed" below find "Lua PMDG_JS41_BTN", press confirm below (ensure this button doesn't conflict with FS key assignments. This button will engage steering from GTN.
5. Again, on the left press "SET", press some other keyboard button, in "Controls sent keys pressed" find "LuaKill PMDG_JS41_BTN", press confirm below. This button will disengage steering from GTN.
6. Go to F1GTN directory, F1GTNConfig.exe, find your aircraft with GTN, press "Configure GTN", select GTN unit on the top (one of the tabs), set "Autopilot" to "Heading hold" mode.
7. To engage steering from GTN, just engage JS41 autopilot with yaw damper, select some pitch mode as required and press Fsuipc button from step 4. There is no need to set CDI mode as "GPS" on GTN specifically, "VLOC" will work too.
4. Do not reference JS41' EHSI as it will not show CDI needle and waypoint info, refer to GTN instead. To disengage, press button from step 5 and use native roll mode as appropriate (HDG/NAV/APR/BC).
5. I noted that sometimes after such manipulations from external units (KLN-90B, GTN, etc.) native HDG mode won't engage properly. To resolve this, just recycle autopilot completely and all roll&pitch modes.
Thanks again Pete for your hint and your great tool!