To create an Opportunity coming from a different record with a default Sale Rep or Partners via Workflow, we can use Workflow Actions Scripts. The Create Record action from a workflow would not work if Team Selling or Multi-Partner Management feature (Setup> Company> Enable Features> CRM tab) is enabled since the Sales Rep and Partners would be a sublist.
Here's a sample script that can create an Opportunity with a default Sales Rep or Partners. The workflow action script is used on a workflow of Record type, "Custom Record".
var opp = nlapiCreateRecord('opportunity');
// custrecord34 is the internal id of the custom field within the custom record that contains the company or customer
// Add a sales rep
// custrecord35 is the internal id of the custom field within the custom record that contains the sales rep id
// "-2" is the internal id for sales rep
// custrecord36 is the internal id of the custom field within the custom record that contains the partner id
var id = nlapiSubmitRecord(opp, true);
Note: If you would like to link the Opportunity on the record that it came from, we would need to set the "Return Type" parameter to "List/Record" and "List/Record" field to "Opportunity" (Customization> Scripting > Scripts > SuiteScript Record > Parameters tab).