How to apply custom header for any single and selected document types (web)?

To apply custom field for single document, please refer to the article linked below:

To add a custom field for multiple documents, please follow the steps given belpw:

Step 1: Go to document settings:

Step 2: Click on Add Custom Fields:

Step 3: Click on add new custom field >> fill in the required details >> Select multiple documents you want custom field to be applicable for:

Step 4: Save: