For third party order, either you can create the material master with item category group as BANS, so the system will automatically pick TAS in the sales order for the material or you can change the item category manually to TAS in the order. 

This will trigger a PR, based on the PR a PO will be generated.

Defining Item Category Group

IMG -> Sales and Distribution -> Sales -> Sales Documents -> Sales Document Item -> Define Item category groups

Defining Item Category -> Sales and Distribution -> Sales -> Sales Documents -> Sales Document Item -> Define Item Categories

Item category TAS
Description 3rd party with SN CM
Item type Blank
Completion rule Blank
Special stock Blank
Relevant for billing B
Billing plan type Blank
Billing block Blank
Pricing X
Statistical value Blank
Revenue recognition Blank
Delimit. start date Blank
Business data item X
Sched. line allowed X
Item relev. for delivery Blank
Returns Blank
Weight/Vol.- relevant X
Credit active X 
Determine cost X
Aut. batch determ. Blank
Rounding permitted Blank
Order qty = 1 Blank
Incomplete proced. 28
PartnerDetermProced T
TextDetermProcedure 01
Item cat. status group 1
Screen seq. group N
Status profile Blank
Create PO autom. Blank
Config. strategy Blank
Mat. variant action Blank
ATP material variant Blank
Structure scope Blank
Application Blank
Value contract material Blank
Contract release ctrl Blank
Repair procedure Blank
Billing form Blank
DIP profile Blank

Assigning Item Category

IMG -> Sales and Distribution -> Sales -> Sales Documents -> Sales Document Item -> Assign Item categories

Creating Material 

MM01

1. On the screen Create Material (Initial Screen) enter the material number if External.

2. Choose Select View(s). (Basic View 1&2, Sales Views, Purchasing views and accounting views).

3. Enter the relevant data and save the material. Use material group BANS in the item material group field.

Creating SD Pricing Conditions for material 

VK11

Creating Vendor Master 

XK01

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