MRP - manage time before production start

Hi gurus,

For my production, I have an external service provider. The issue is that the service provider has other customers. When I send an order, I have to wait about 3 weeks until the production really start.

I would like to know how I can manage this time in SAP.

I would like MRP to create purchase requisitions (for example) 3 weeks after the creation of the order for the service provider, so when the production really start.

Thank you in advance,