The common way is to populate it with the ID of the record being looked up. Another way (I think) is to populate the field with the value of the External ID from the record being looked up and Salesforce will automatically resolve that to the correct record. Reading the section "upsert () and Foreign Keys" in the API guide leads me to think this.
Full Answer
How do I set up a lookup filter in Salesforce?
Click Show Filter Settings in the Lookup Filter Section. Click in the field box and choose Current Record then the field to filter on (e.g., Technology Partner). Choose equals for the operator and Field for the Value/Field.
What is a lookup field in Salesforce?
When creating a lookup field you are essentially relating the field to another object (e.g, table) for its values. They can be used to lookup a value on related lists, be useful for relating objects for custom report types, or create a relationship for tracking to name a few.
How do I create an equal to value in Salesforce?
Choose equals for the operator and Field for the Value/Field. Because we are using a Field as the equal to value, click in the value box and choose Current Lookup then the lookup name and then the AccountID field. Choose if the field will require a value.
What is the difference between external ID and unique ID in Salesforce?
Here are the differences between External IDs and Unique IDs for fields on Salesforce records. The 'External ID' field may be used to reference an ID from another, external system. External IDs are searchable in Salesforce and you can also use the Upsert API call with the External ID to refer to records when making changes via Data Loader.
Can we make lookup field unique in Salesforce?
You can achieve this one by creating a Text(unique) field and a workflow rule. 1. Create a Text field (Unique) on Custom Object. Make sure to check unique checkbox while creating a text field.
Can a lookup field have multiple values in Salesforce?
You can only have one related record per Lookup field.
How do I create a multi select lookup field?
To create a multi-select lookup field Go to Setup > Customization > Modules and Fields. Click the required module, in this case, Prospects. From the list of fields available, drag and drop the Multi-select lookup field. Note that you can create a maximum of 2 Multi-select lookup fields per module.
Can we create multi select lookup field in Salesforce?
No it does not seems possible on standard layout. You can develop a custom lightning component for multi select. For storing multiple record, you will have to create a junction object instead of lookup field to hold all selected records.