tuplets can't cross barlines

Hello there,
I've a midi file that I want to add to a score, I open the midi file then i select all notes and I paste them into the score but I have this error message : tuplets can't cross barlines
I've seeing this answer " If you copy starting from the pickup to a new score without a pickup, then everything is off by a beat, and the tupelts do indeed cross barlines."
But how to add a pickup I'm a newbie to musescore

Thanks in advance