First create a new custom SharePoint list in Office 365 with the following properties:

Typ = Custom List

Name = Purchasing Requests

Add the following columns to the list:

Material: Single line of text (rename the default Title column to Material), required

Plant: Single line of text, required

Quantity: Number, required

Delivery Date: Date only, required

Cost Center: Single line of text, required

Purchase Requisition ID: Single line of text

After that navigate to List-> Settings > Nintex Workflow and select Create new workflow. A new list workflow related to the above created SharePoint list will be created then.

First add the following three workflow actions in the Nintex Workflow Designer:

Assign (from the User interaction group)

Create dictionary (from the Operations group)

Call HTTP web service (from the Integration group)

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Configure the Request approval action with the following properties:

Participant = ApprovalManagers (group of approvers)
Task Title = Approve Purchase Requisition
Related Item = Current Item

All the other settings are optional. Confirm your entries with Save.

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Configure the Create dictionary action:

The dictionary contains the header data for the web service call. We highly recommend to test the web service in a REST client before trying to call it in a Nintex Workflow. More detailed information about that can be found here.

These entries are required:

Authorization: APIKEY [BASE64 encoded ECS Core API-key]
Accept: application/json
Content-Type: application/json

Save the dictionary in workflow variable RequestHeader.

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After that configure workflow action Call HTTP web service.

The following entries have to be set:

Adress: Web service URL including input parameters. For the parameter values insert references to the corresponding columns of the SharePoint list.

Request Type: POST

RequestHeaders: workflow variable RequestHeader
Response Content: workflow variable ResponseContent
Response Status Code: workflow variable Result

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The workflow would be fully functional by now but in this scenario the document number of the newly created purchase requisition should be written back to the SharePoint list.
In the web service, return parameter ReqItemID has been created therefore and is displayed in the response content.

Add the following workflow actions to your workflow:

Get an Item from a dictionary (from the Operations group)
Set field in current item (from the Libraries and Lists group)

Optionally you may also add the Log in history list action between both workflow actions:

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First configure the Get an Item from a dictionary action.

Dictionary: Workflow variable ResponseContent
Item name or path: result (This has the effect that only the result part of the dictionary that contains the document number will be extracted)
Output: Workflow variable RequestItemID

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Optionally you may log workflow variable RequestItemID in the workflow history list.

With workflow action Set field in current item we want to write the document number in workflow variable RequestItemID back to the SharePoint list:

Field: Purchase Requisition ID
Value: Workflow variable RequestItemID

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Save your entries and publish the workflow.