GSEA enrichr with 10x genomics differential_expression ranks
You should be using the most positive fold changes, not the most negative. The positive ones are the ones that are most highly expressed in your cluster of interest. You probably want to use less than 100 genes. Depending on the experiment, you may not have that many good markers for a particular cluster.
The other question you mentioned refers to DESeq2 specifically, so it is not applicable here.