Transit gateway vpc attachment argument depends_on

I am using CDKTF to create VPCs and connect these to a shared transit gateway, mostly
based on this sample

I found no place describing depends_on argument to ec2_transit_gateway_vpc_attachment

Ideally I would like to supply IDs of the objects the attachment depends on. Instead CDKTF expects for these to be cdktf.ITerraformDependable. The latter is never described btw.

Can I make a string cdktf.ITerraformDependable by just supplying a method

def fqn(self) -> builtins.str: