I presume you mean the pink flower? Does the ferny foliage belong to it? If it does I think it is a cosmos. You just let the flower go to seed and then cut the seed head off, put it in a paper bag and when the seed head is dry you can get the seed from it. The important part of seed collecting is to make sure the seed head is dry or else it will go mouldy and the seeds will be no good.
Well spotted the foliage -it is to a Cosmos, but the foliage does not belong to this plant, It was in a pollinator mix I purchased, but it's not in the listed plants that was included in that mix.
Looks like malva sylvestris, it will form a seed head when the petals fall off. Mine never survived the seedling stage. Someone else on the forum will be along shortly with the answer.
Life's tragedy is that we get old too soon and wise too late.
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The important part of seed collecting is to make sure the seed head is dry or else it will go mouldy and the seeds will be no good.