For those of you on PC using the freeware PMS GTN750 toolbar tool allows you access to set squawk on it (remember to enable it for alt reporting too). The squawk code won’t appear on the 737, but it will set squawk which ATC will then recognise. You can then just hide the GTN panel again using CTRL+SHIFT+S.
EDIT: If you want to make squawk code appear after using the GTN750 to enter it, switch the transponder from ch 1 → 2 and back again. The entered squawk code in the GTN750 should now appear in the 737 transponder panel display. Also don’t forget to turn the transponder ON from standby and select TA/RA.