Whenever referring to ID's you should provide examples.

Looking at the XML entries for UniRef50 you can see what the ID's are the same (since they come from UniProt). Word UniRef90_ or UniRef50_ is pre-pended to the actual UniProt Accession.

<entry id="UniRef50_A0A5A9P0L4" updated="2022-12-14">    <---------------- UniRef50
<name>Cluster: peptidylprolyl isomerase</name>
<property type="member count" value="1"/>
<property type="common taxon" value="Triplophysa tibetana"/>
<property type="common taxon ID" value="1572043"/>
<property type="GO Molecular Function" value="GO:0005524"/>
<property type="GO Molecular Function" value="GO:0005509"/>
<property type="GO Molecular Function" value="GO:0003755"/>
<property type="GO Molecular Function" value="GO:0004672"/>
<property type="GO Biological Process" value="GO:0006468"/>
<property type="GO Cellular Component" value="GO:0005783"/>
<representativeMember>
<dbReference type="UniProtKB ID" id="A0A5A9P0L4_9TELE">
<property type="UniProtKB accession" value="A0A5A9P0L4"/>
<property type="UniParc ID" value="UPI0012572FB4"/>
<property type="UniRef100 ID" value="UniRef100_A0A5A9P0L4"/>             <---- UniRef100
<property type="UniRef90 ID" value="UniRef90_A0A5A9P0L4"/>

If your list is extensive, consider using UniProt id-mapping, which can handle up to 100,000 items in a single run. Since the clusters are updated periodically, you might want to find the corresponding mapping when you acquired the list of UniRef90 from here. Or get the XML file and search your UniRef90 and it’s UniRef50 ids as shown in GenoMax reply.