Hello Ziv Markovich,
I use an FM and the following syntax extensively in user-exit IWO10009 to update fields at Order Save event. For example, if I want to update Order Short text then, the code is:
CAUFVD_IMP-KTEXT = 'Your New text is here'. CALL FUNCTION 'CO_IH_SET_HEADER' EXPORTING CAUFVD_IMP = CAUFVD_IMP. PERFORM HEADER_UPDATE(SAPLCOIH).
This modifies the Order short text to 'Your New text is here'.
Here is another such example in my earlier replies.: Dates in Maintenance Order
Hope this helps you.
Regards
KJogeswaraRao