HLS var_stream_map for vtt

[mars]

Hello, I'm trying to find a solution to create a master playlist for hls. If I run the command from the documentation

`
ffmpeg -y -i input_with_subtitle.mkv \

-b:v:0 5250k -c:v h264 -pix_fmt yuv420p -profile:v main -level 4.1 \
-b:a:0 256k \
-c:s webvtt -c:a mp2 -ar 48000 -ac 2 -map 0:v -map 0:a:0 -map 0:s:0 \
-f hls -var_stream_map "v:0,a:0,s:0,sgroup:subtitle,sname:English" \
-master_pl_name master.m3u8 -t 300 -hls_time 10 -hls_init_time 4 -hls_list_size \
10 -master_pl_publish_rate 10 -hls_flags \

delete_segments+discont_start+split_by_time ./tmp/video.m3u8
`

it works correctly. But when I try to do the same using api

`
av_opt_set_int(ofmt_ctx, "hls_list_size", 6, AV_OPT_SEARCH_CHILDREN);
av_opt_set(ofmt_ctx, "hls_flags", "omit_endlist+append_list+delete_segments", AV_OPT_SEARCH_CHILDREN);
av_opt_set_int(ofmt_ctx, "hls_time", 5, AV_OPT_SEARCH_CHILDREN);
av_opt_set(ofmt_ctx, "master_pl_name", "hls.m3u8", AV_OPT_SEARCH_CHILDREN);
av_opt_set(ofmt_ctx, "fix_sub_duration", "1", AV_OPT_SEARCH_CHILDREN);
av_opt_set(ofmt_ctx, "var_stream_map", "v:0,a:0,s:0,sgroup:subs", AV_OPT_SEARCH_CHILDREN);
`

And I set the sgroup:subs options so that the #EXT-X-MEDIA:TYPE=SUBTITLES tag is generated in the manifest file, then an error occurs

[hls @ 0x55c4c4b3e580] Invalid keyval sgroup:subtitle
[hls @ 0x55c4c4b3e580] Variant stream info update failed with status ffffffea

Please tell me how to correctly pass var_stream_map to ofmt_ctx so that the manifest is created correctly