In your first post, you say a 'Free Synth VST', so I'm guessing you are using a DAW for the Audio playback to your Video, or maybe considering it? If so, and your DAW doesn't have any Synth VSTi's,
just Google for free VST Synth's etc...
Like these:
Best 20 Free Synth VST/AU Plugins of 2021! - The Home Recordings
The best free VST synth plugins 2021: wavetable, FM, virtual analogue and more | MusicRadar
If you don't want it to sound totally synthy and lean more towards Bass Guitar sounding, you can double up.
Try Recording the Bass line in question as a midi Bass Synth part on one track, and then, record another track with just a dry Electric Bass part doubling the Synth.
(You could even move the Synth part up an Octave). Then mix in the Synth track just under the Bass Guitar track. Buss them together if needed (including a Comp on the buss track to glue both parts together).
There are plenty of YouTube tutorial videos demonstrating this technique.
EDIT: Out of interest, are you going to be miming to playback for your video? If not, and you are playing live, ignore all that I said....