Hi,
I presume you mean in a live/ recording performance, so you have'nt recorded anything in advance.
Analog Lab it self have no sequencer and arpeggiator. The presets react like they are programmed. Depending on the original applications you may have different ways to trigger the sequence. You need full individual applications to change settings like that. This can perhaps give options for a few applications only, but i can't tell for sure. Perhaps mostly, if you also use a DAW/ host.
I assume you play a note on a keyboard controller, when you say "start the sequencer".
It sound like your external sequencer have a start button.
If the external sequencers start button has a "keyboard on" feature, so it can start, when you play a note on your controller, then that should be it.
Alternatively the external sequencer perhaps can activate notes for your Analog Lab preset.
And then you have to sync the tempo in one way or another.
