I have the FT-991 using RigCat to the rig. The IF WIDTH CAT commands to the two rigs differ.
I guess you could make or modify your own FT-891 xml file by looking at the FT-991.xml and:
the FT-891 CAT manual: link, + the FT-991 CAT manual: link
In FT-891 the IF WIDTH set command seems to be SH01XX; where XX is 2 digits from a table in the manual, while FT-991 has SH0XX. I set up the WIDTH command thus: NA01;SH002; (narrow, 100 Hz),... NA00;SH011 (wide, 800 Hz), ... etc in my FT-991.xml för RigCAT in FLdigi. Don't know if FLrig needs the same.
You can take a look at my expanded RigCat FT-991-v2.xml here. It contains some enhancements for use in FLdigi including my init section.
Good luck with FT-891.
Poul / SA7CND