You need to paste in Header Area:
{
"Accept": "application/json; odata=verbose",
"IF-MATCH": "*",
"X-HTTP-Method": "MERGE",
"content-type": "application/json; odata=verbose"
}
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In BODY area you need to paste it:
{
'__metadata':{
'type': 'SP.Data.DummyDocLibItem'
},
'[Your Hyperlink Column Name]':
{
'__metadata':{
'type':'SP.FieldUrlValue'
},
'Description': '',
'Url': ''
}
}