If you really want the player to spawn in with an item you can use the Player joins the world global trigger. Use the Has (Event/target entity) in inventory (item) block from the Player procedures tab to check if the player already has the item in their inventory and if they don't, add it to their inventory.
Since you can't craft the custom portal igniter, i am looking for a way for the player to spawn with
The portal igniter is called custom:dimension. It's a big M in the item selector menu.
If you really want the player to spawn in with an item you can use the Player joins the world global trigger. Use the Has (Event/target entity) in inventory (item) block from the Player procedures tab to check if the player already has the item in their inventory and if they don't, add it to their inventory.
Image of it?