How Do I Place A Block From The Players Mainhand

I have this custom block and on a right click I want to:

• check wether the item the players is holding in his mainhand is a slab
• if this is the case, I want to place this item

To achieve this I went to: custom_block > triggers > on block right clicked, and did:

I will be replacing "logs" with "slab" in the future, I however can't figure out how to place the item (block) the players is holding in his mainhand.

Thanks in advance!