# video concatenates, no audio from stream
# stream looks bad
ffmpeg -i intro.ts -i - -lavfi "[0:v][1:v]scale2ref[wm][base];[wm]setsar=1[wm];[base][wm]concat=2" output.ts

# intro audio is played over stream video
# video does not concatenate
# gives messages like:
# [Parsed_concat_2 @ 0x562f943bfee0] Buffer queue overflow, dropping.bitrate=10649.7kbits/s speed=0.468x
#    Last message repeated 17 times
streamlink -o - https://www.youtube.com/watch?v=IYPgZFGNpYY best | ffmpeg -i intro.ts -i - -lavfi "[0:v][1:v]scale2ref[wm];[wm]setsar=1[wm];[0:v][wm]concat=2" output.ts