Power BI LookupValue

LOOKUPVALUE(<result_columnName>, <search_columnName>, <search_value>[, <search_columnName>, <search_value>]…[, <alternateResult>]) 

Example:

Partner = LOOKUPVALUE(DimPartners[PartnerName],DimPartners[PartnerID],RELATED(DimProjectActivities[PartnerID]))

Links:
https://docs.microsoft.com/en-us/dax/lookupvalue-function-dax