Special Procurement key for STO

Hi experts,

We have three plants say 1000, 1100, 1200.

Material "A" will be received to Plant 1000 from both plants 1100 and 1200 through stock transfer order(STO).

In this case how can we create special procurement key for material "A" for 1000 plant, How should I map this scenario to consider two plants are as sender plants one plant as receiver plant. What are the settings required to suit this requirement.

Please help in this issue

Thanks in advance.